Introduction to aciclovir
Aciclovir, also known as acyclovir, is a synthetic nucleoside analogue antiviral medication first developed in the late 1970s and approved in 1982. It was one of the first highly selective antiviral agents, paving the way for modern antiviral pharmacology. Aciclovir is used for infections caused by herpes simplex virus types 1 and 2 and varicella-zoster virus. Available in oral tablets, capsules, suspension, topical cream, and intravenous injection, it works by selectively inhibiting viral DNA polymerase after activation by viral thymidine kinase. This selectivity provides a favorable safety profile with minimal toxicity to uninfected cells. The drug is most effective when initiated early in the course of infection, ideally within 24-48 hours of symptom onset. Aciclovir is generally well tolerated, with common side effects being gastrointestinal disturbances and headache. The availability of generic aciclovir has made it one of the most accessible antiviral medications worldwide, and it is included on the WHO Model List of Essential Medicines. From Happy Family Store
Mechanism of action
The antiviral activity of aciclovir is mediated through a highly selective, multi-step mechanism that specifically targets virus-infected cells. The first and critical step in the activation of aciclovir is phosphorylation to aciclovir monophosphate, a reaction catalyzed by viral thymidine kinase (TK), an enzyme encoded by HSV and VZV that is expressed only in virus-infected cells. In uninfected cells, cellular thymidine kinases have minimal activity against aciclovir, so the drug remains largely in its inactive form. This selectivity is the fundamental basis for aciclovir’s favorable therapeutic index, as the drug is preferentially activated at the site of viral replication. Once formed, aciclovir monophosphate is further phosphorylated to aciclovir diphosphate and then to aciclovir triphosphate by cellular guanylate kinase and other cellular kinases, respectively. Aciclovir triphosphate, the active moiety, accumulates to high concentrations in virus-infected cells, reaching levels 40-100 times higher than in uninfected cells. Aciclovir triphosphate exerts its antiviral effect through two mechanisms. First, it competitively inhibits viral DNA polymerase, the enzyme responsible for replicating viral DNA. Aciclovir triphosphate binds to the DNA polymerase with a much higher affinity for the viral enzyme than for cellular DNA polymerases, further contributing to its selectivity. Second, aciclovir triphosphate is incorporated into the growing viral DNA chain as a substrate for DNA polymerase. Because aciclovir lacks the 3-hydroxyl group necessary for forming the phosphodiester bond with the next nucleotide, its incorporation results in absolute chain termination, preventing further elongation of the viral DNA strand. This chain termination is irreversible, and the terminated DNA chain cannot be extended even if additional nucleotides are available. The combination of competitive inhibition of DNA polymerase and chain termination of the growing viral DNA strand results in potent inhibition of viral DNA synthesis, effectively halting viral replication. The selectivity of aciclovir is further enhanced by the presence of a viral enzyme that can remove aciclovir monophosphate from terminated DNA chains. The viral exonuclease activity associated with HSV DNA polymerase can excise the terminal aciclovir monophosphate residue, but this process is inefficient and occurs at a much slower rate than the incorporation of additional aciclovir triphosphate molecules. The net effect is a gradual accumulation of terminated DNA chains within the infected cell. The spectrum of antiviral activity of aciclovir correlates with the presence and efficiency of viral TK in different herpesviruses. HSV-1 and HSV-2 have highly efficient TK enzymes that rapidly phosphorylate aciclovir, making these viruses the most susceptible. VZV TK is somewhat less efficient, requiring higher concentrations of aciclovir for equivalent antiviral activity. EBV and CMV do not encode a TK enzyme, and their susceptibility to aciclovir is limited, relying on alternative phosphorylation pathways that are much less efficient. For EBV, aciclovir has some activity against the lytic phase of infection but no activity against latent infection. For CMV, aciclovir has minimal activity, and alternative agents such as ganciclovir and valganciclovir are preferred. The pharmacokinetics of aciclovir involve relatively poor oral bioavailability, with only 15-30% of an orally administered dose reaching the systemic circulation. This limitation led to the development of valacyclovir, the L-valyl ester prodrug of aciclovir, which has approximately 3-5 times higher oral bioavailability. Aciclovir is widely distributed throughout the body, including into the cerebrospinal fluid, and it crosses the placenta. The drug is eliminated primarily by renal excretion through both glomerular filtration and tubular secretion, with a half-life of approximately 2.5-3.5 hours in adults with normal renal function. Dose adjustment is necessary in patients with impaired renal function to prevent accumulation and potential toxicity. The unique mechanism of action of aciclovir, with its requirement for viral TK for activation and its selective inhibition of viral DNA polymerase, provides a paradigm for the development of selective antiviral agents and remains a foundation of herpesvirus therapy.
Therapeutic indications
Aciclovir is indicated for the treatment and prophylaxis of infections caused by herpes simplex virus types 1 and 2 and varicella-zoster virus. In the management of genital herpes, aciclovir is used for three distinct purposes: treatment of first-episode infections, treatment of recurrent episodes, and chronic suppressive therapy to reduce the frequency of recurrences. First-episode genital herpes typically presents with multiple painful vesicular lesions on the genital area, often accompanied by systemic symptoms such as fever, headache, and myalgia. Oral aciclovir 200 mg five times daily or 400 mg three times daily for 7-10 days reduces the duration and severity of symptoms, accelerates healing, and reduces viral shedding. Recurrent genital herpes involves milder, shorter episodes, and episodic treatment with aciclovir 200 mg five times daily or 400 mg three times daily for 5 days, initiated at the first sign of recurrence, reduces symptom duration by 1-2 days. For patients with frequent recurrences (6 or more episodes per year), chronic suppressive therapy with aciclovir 400 mg twice daily reduces recurrence rates by 70-80% and improves quality of life. Suppressive therapy can be continued for extended periods, with reassessment of the recurrence rate annually. Orolabial herpes (cold sores), caused primarily by HSV-1, is treated with oral aciclovir 200 mg five times daily for 5 days, initiated at the first sign of a lesion (tingling, burning, or itching). Topical aciclovir cream 5% applied five times daily for 4-5 days is also effective for cold sores, though it is less effective than oral therapy for established lesions. For patients with frequent or severe cold sores, oral suppressive therapy may be considered. Herpes simplex keratitis, an infection of the cornea that can lead to corneal scarring and visual impairment, is treated with topical aciclovir ophthalmic ointment 3% five times daily. This formulation is not available in all countries, and alternative ophthalmic antiviral preparations may be used where aciclovir ointment is not available. Varicella (chickenpox) in otherwise healthy children is typically a self-limited illness, but aciclovir therapy initiated within 24 hours of rash onset reduces the duration and severity of symptoms, including fever and the number of lesions. The recommended dose is 20 mg/kg (up to 800 mg) four times daily for 5 days. Aciclovir is particularly recommended for varicella in adolescents, adults, pregnant women, and immunocompromised patients, who are at higher risk for severe disease and complications. Herpes zoster (shingles), caused by reactivation of latent VZV, is treated with oral aciclovir 800 mg five times daily for 7-10 days. Early initiation of therapy, within 72 hours of rash onset, reduces the duration of acute pain, accelerates lesion healing, and may reduce the incidence and severity of postherpetic neuralgia, a chronic pain condition that can persist for months after the rash resolves. Higher doses are required for VZV than for HSV because of the lower sensitivity of the VZV TK enzyme for aciclovir. In immunocompromised patients, including those with HIV infection, organ transplantation, or hematologic malignancies, herpesvirus infections can be more severe and prolonged. Intravenous aciclovir is often required for initial treatment, followed by oral therapy for consolidation and suppressive therapy to prevent recurrence. Prophylactic use of oral or intravenous aciclovir is standard practice in certain transplant populations to prevent HSV and VZV reactivation during periods of intense immunosuppression. The topical cream formulation of aciclovir is also available over the counter in many countries for the treatment of cold sores, providing accessible treatment for this common condition. The breadth of indications for aciclovir reflects its central role for herpesvirus infections across diverse patient populations and clinical settings.
Dosage and administration
The dosage of aciclovir varies depending on the indication, the formulation used, patient age and weight, and renal function. For oral aciclovir, the standard adult dose for first-episode genital herpes is 200 mg five times daily (approximately every 4 hours, omitting the nighttime dose) or 400 mg three times daily for 7-10 days. For recurrent genital herpes, 200 mg five times daily or 400 mg three times daily for 5 days is recommended, with treatment initiated at the first sign of recurrence. For chronic suppressive therapy of genital herpes, the standard dose is 400 mg twice daily. For orolabial herpes, 200 mg five times daily for 5 days is recommended. For varicella in immunocompetent patients, the dose is 20 mg/kg (up to 800 mg) four times daily for 5 days. For herpes zoster, 800 mg five times daily for 7-10 days is recommended. The oral suspension (200 mg/5 mL or 400 mg/5 mL) is available for patients who have difficulty swallowing tablets, including children and elderly patients. For pediatric patients, the dose of oral aciclovir for varicella is 20 mg/kg (up to 800 mg) four times daily for 5 days. For herpes simplex infections, the pediatric dose is 10-20 mg/kg per dose (up to 400 mg) three to five times daily, depending on the indication. For neonatal herpes simplex infections, intravenous aciclovir is typically used under specialist supervision. For patients with renal impairment, the dose of aciclovir must be adjusted to prevent accumulation and potential toxicity. Aciclovir is eliminated primarily by renal excretion, and the dosing interval should be extended based on creatinine clearance. For patients with creatinine clearance of 25-50 mL/min, the standard dose is given every 8 hours. For patients with creatinine clearance of 10-25 mL/min, the dose is given every 12 hours. For patients with creatinine clearance less than 10 mL/min, the dose is given every 24 hours. Patients undergoing hemodialysis should receive the dose after dialysis, as aciclovir is effectively removed by the procedure. Topical aciclovir cream 5% is applied to cold sores five times daily for 4-5 days. A thin layer of cream should be applied to cover the entire lesion and the surrounding area. Treatment should be initiated at the first sign of a cold sore, ideally during the prodromal phase when tingling, burning, or itching is first noticed. The cream should be applied using a clean fingertip or a cotton swab, and hands should be washed before and after application to prevent spread of the virus. The cream formulation is for external use only and should not be applied to mucous membranes or the eyes. Topical aciclovir ophthalmic ointment 3% is applied to the lower conjunctival sac five times daily for herpes simplex keratitis. The ointment should be applied every 4 hours, and treatment should continue for at least 3 days after the cornea has healed. This formulation should be used only under the supervision of an ophthalmologist. Intravenous aciclovir is administered as a slow infusion over at least one hour to prevent renal tubular precipitation of the drug. The dose for mucocutaneous HSV infections in immunocompromised patients is 5 mg/kg every 8 hours for 7 days. For HSV encephalitis, the dose is 10-15 mg/kg every 8 hours for 14-21 days. For neonatal HSV infections, the dose is 10-20 mg/kg every 8 hours for 14-21 days. For VZV infections in immunocompromised patients, the dose is 10-15 mg/kg every 8 hours for 7-10 days. Adequate hydration should be maintained during intravenous aciclovir therapy to reduce the risk of nephrotoxicity. The infusion should be administered over at least one hour, and the patient should be well hydrated before and after the infusion. The intravenous formulation should be used with caution in patients with pre-existing renal impairment, and dose adjustment is necessary based on creatinine clearance. The choice of formulation and dosing regimen should be individualized based on the severity of the infection, the immune status of the patient, renal function, and the ability to adhere to the prescribed regimen. Adherence to the full course of therapy is important for achieving optimal outcomes, even if symptoms improve before the medication is completed.
Side effects and adverse reactions
Aciclovir is generally very well tolerated, with a favorable safety profile that contributes to its widespread use. The nature and severity of adverse effects depend on the route of administration, the dose, and the duration of therapy. For oral aciclovir, the most commonly reported side effects are gastrointestinal and neurological. Nausea, vomiting, diarrhea, and abdominal pain occur in approximately 5-15% of patients, particularly at higher doses such as those used for herpes zoster (800 mg five times daily). These symptoms are usually mild and may be reduced by taking the medication with food. Headache is also common, occurring in 10-15% of patients in some studies. Dizziness, fatigue, and malaise have been reported less frequently. Skin rashes, including urticaria and photosensitivity, occur rarely. At the high doses used for herpes zoster, central nervous system effects including confusion, hallucinations, agitation, somnolence, and seizures have been reported, primarily in elderly patients, those with renal impairment, and those receiving concurrent nephrotoxic medications. These neurotoxic effects are thought to result from accumulation of the drug and its metabolites in the central nervous system. The risk can be minimized by appropriate dose adjustment for renal function and adequate hydration. For topical aciclovir cream, adverse effects are limited to mild local reactions. Burning, stinging, itching, and dryness at the application site occur in approximately 5-10% of patients. These reactions are typically mild and transient. Allergic contact dermatitis has been reported rarely and is confirmed by patch testing. The cream formulation contains propylene glycol and other excipients that may cause irritation in sensitive individuals. The ophthalmic ointment may cause transient stinging, burning, and blurred vision immediately after application. Punctate keratitis and follicular conjunctivitis have been reported rarely. Patients using the ophthalmic ointment should be advised that temporary visual blurring is normal and should not drive or operate machinery until vision clears. For intravenous aciclovir, adverse effects are more common and potentially more serious. The most significant concern is nephrotoxicity, which results from precipitation of aciclovir crystals in the renal tubules, particularly when the drug is administered as a rapid bolus or in patients with pre-existing renal impairment, dehydration, or concurrent use of other nephrotoxic medications. Nephrotoxicity can manifest as acute kidney injury with elevated serum creatinine, oliguria, and, in severe cases, acute renal failure. The risk can be minimized by administering the infusion over at least one hour, ensuring adequate hydration, and adjusting the dose for renal function. Most cases of nephrotoxicity are reversible upon discontinuation of the drug or dose adjustment. Neurotoxicity is the second major concern with intravenous aciclovir. Symptoms range from mild confusion, tremors, and myoclonus to severe encephalopathy, seizures, and coma. The risk is highest in elderly patients, those with renal impairment, those receiving high doses, and those with pre-existing central nervous system disease. Neurotoxicity is generally reversible within days to weeks after discontinuation of the drug. Hemodialysis effectively removes aciclovir and its metabolites and may be used in severe cases. Other adverse effects of intravenous aciclovir include phlebitis at the infusion site, nausea and vomiting, and liver enzyme elevations. Hypersensitivity reactions, including anaphylaxis, angioedema, urticaria, and rash, are rare but can occur with any formulation. Hematologic effects, including thrombocytopenia, leukopenia, and neutropenia, have been reported rarely. The long-term safety of aciclovir has been evaluated in patients receiving chronic suppressive therapy for genital herpes for periods exceeding 10 years. These studies have shown that aciclovir is safe and well tolerated during long-term use, with no evidence of cumulative toxicity or an increased risk of cancer or other adverse outcomes. The favorable long-term safety profile supports the use of aciclovir for extended periods when clinically indicated. Patients should be counseled about potential side effects and advised to report any persistent or severe symptoms to their healthcare provider. Patients receiving intravenous aciclovir should be monitored for changes in renal function and neurological status, and appropriate dose adjustments should be made when necessary.
Drug interactions
Aciclovir has relatively few clinically significant drug interactions compared to many other medications. Its interaction profile is primarily related to its renal elimination and the potential for additive nephrotoxicity when combined with other nephrotoxic drugs. The most clinically important interaction is with probenecid, a medication used to treat gout and to increase the plasma concentrations of certain antibiotics. Probenecid competes with aciclovir for renal tubular secretion, reducing the clearance of aciclovir by approximately 20-30% and increasing its half-life. While this interaction is not usually clinically significant for oral aciclovir, it may increase the risk of aciclovir toxicity when high doses are used or in patients with pre-existing renal impairment. Dose adjustment of aciclovir may be necessary in patients receiving concurrent probenecid. Concomitant use of aciclovir with other nephrotoxic medications increases the risk of renal toxicity. Medications that may enhance the nephrotoxic potential of aciclovir include aminoglycoside antibiotics (gentamicin, tobramycin, amikacin), cyclosporine, tacrolimus, amphotericin B, nonsteroidal anti-inflammatory drugs (particularly in high doses or in elderly patients), and certain chemotherapeutic agents (cisplatin, methotrexate). Patients receiving these combinations should be monitored closely for changes in renal function, and appropriate dose adjustments of all nephrotoxic agents should be made as necessary. The combination of aciclovir with drugs that affect the central nervous system may increase the risk of neurotoxicity. This is of particular concern with intravenous aciclovir and with the high oral doses used for herpes zoster. Caution should be exercised when aciclovir is coadministered with other medications that can cause central nervous system depression or excitation, including opioids, benzodiazepines, antipsychotics, and antidepressants. The interaction with zidovudine (AZT), an antiretroviral medication used in the treatment of HIV, warrants mention. Both aciclovir and zidovudine can cause neurotoxicity and hematologic toxicity, and the combination may increase the risk of these adverse effects. Patients receiving both medications should be monitored for signs of neurotoxicity (confusion, lethargy, seizures) and hematologic toxicity (anemia, neutropenia). Aciclovir may reduce the efficacy of live attenuated varicella vaccine (varicella vaccine) and herpes zoster vaccine (shingles vaccine). Because aciclovir inhibits VZV replication, it may interfere with the immune response to vaccines containing live VZV. The vaccines should be administered at least 24 hours before starting aciclovir therapy or, if the patient is already receiving aciclovir, the vaccine should be deferred until at least 24 hours after the last dose of aciclovir. Inactivated vaccines (such as the recombinant shingles vaccine) are not affected by aciclovir. Aciclovir has no known interactions with oral contraceptives, anticoagulants, or antidiabetic agents, and no significant interactions with food, alcohol, or herbal products. Patients can use aciclovir without dietary restrictions. However, patients should be advised to maintain adequate hydration during aciclovir therapy, particularly at high doses, to reduce the risk of nephrotoxicity. Caffeine and other methylxanthines have no known interaction with aciclovir. The favorable drug interaction profile of aciclovir is one of its advantages, particularly in elderly patients and those with multiple medical conditions who may be taking several medications concurrently. However, healthcare providers should still conduct a thorough medication review before prescribing aciclovir, with particular attention to the use of other nephrotoxic medications and the potential for additive neurotoxicity. In most cases, aciclovir can be safely used with the majority of other medications without clinically significant interactions, making it a versatile and practical choice for antiviral therapy in many patients.
Contraindications and precautions
Aciclovir is contraindicated in patients with known hypersensitivity to aciclovir, valacyclovir, or any component of the formulation. Cross-reactivity between aciclovir and valacyclovir is expected, as valacyclovir is converted to aciclovir in the body. Patients with a history of allergic reactions to either drug should not receive aciclovir. There are no other absolute contraindications to aciclovir, but several precautions must be observed to ensure safe use. Renal precautions are paramount, as aciclovir is eliminated primarily by renal excretion and can cause nephrotoxicity. The dose must be adjusted based on creatinine clearance, and adequate hydration should be maintained throughout therapy to reduce the risk of crystal nephropathy. Patients with pre-existing renal impairment, including elderly patients with age-related decline in renal function, are at increased risk for both nephrotoxicity and neurotoxicity. Creatinine clearance should be calculated before initiating therapy, and the dosing interval should be extended as indicated. During intravenous aciclovir therapy, renal function should be monitored at baseline and regularly throughout treatment, particularly in patients with pre-existing renal impairment, those receiving high doses, and those receiving concurrent nephrotoxic medications. Neurological precautions are important, particularly in elderly patients, those with renal impairment, those with pre-existing central nervous system disease, and those receiving high doses of aciclovir. Patients should be monitored for signs of neurotoxicity, including confusion, hallucinations, agitation, tremors, myoclonus, and seizures. If neurotoxic symptoms develop, aciclovir should be discontinued or the dose reduced, and the patient should be evaluated for alternative causes of the neurological symptoms. Hemodialysis can rapidly reduce aciclovir concentrations and may be indicated in severe cases of neurotoxicity. Hepatic precautions are less critical, as aciclovir is not metabolized by the liver and hepatotoxicity is rare. However, liver function tests should be monitored in patients receiving prolonged intravenous therapy or in those with pre-existing liver disease. The safety of aciclovir during pregnancy has been evaluated in large observational studies and pregnancy registries. Data from the Acyclovir Pregnancy Registry and other studies involving over 1,000 pregnant women exposed to aciclovir during the first trimester have not shown an increased risk of major birth defects compared to the general population. Aciclovir is classified as FDA Pregnancy Category B (prior to the 2015 labeling changes), indicating that animal studies have not demonstrated fetal risk but controlled human studies are lacking. The drug is commonly used during pregnancy for the treatment of genital herpes, particularly during the third trimester to reduce the risk of HSV transmission to the neonate during delivery. The Centers for Disease Control and Prevention recommends that pregnant women with a history of genital herpes be offered suppressive aciclovir therapy beginning at 36 weeks of gestation to reduce the risk of recurrence at delivery and the need for cesarean section. Aciclovir is excreted into breast milk in small amounts, but the concentration is insufficient to cause adverse effects in the nursing infant. The American Academy of Pediatrics considers aciclovir to be compatible with breastfeeding. However, caution should be exercised in nursing mothers of infants with renal impairment, as the drug could theoretically accumulate. In pediatric patients, aciclovir is used in all age groups, including neonates, for the treatment of herpes simplex and varicella-zoster infections. Neonates have immature renal function, and the dosing of intravenous aciclovir must be carefully calculated and adjusted for gestational age and postnatal age. The oral suspension formulation is suitable for children who cannot swallow tablets. The safety of aciclovir in children has been established through extensive clinical use. In elderly patients, age-related decline in renal function is the primary consideration. Creatinine clearance should be estimated, and the dose adjusted accordingly. Elderly patients are also more susceptible to the neurotoxic effects of aciclovir, particularly at high doses. Adequate hydration is especially important in this population. In immunocompromised patients, including those with HIV infection, organ transplantation, or hematologic malignancies, herpesvirus infections can be more severe and prolonged. These patients may require higher doses, longer treatment durations, and more aggressive monitoring for adverse effects. Resistance to aciclovir is more common in immunocompromised patients, particularly those with advanced HIV infection or those receiving prolonged or repeated courses of therapy. In patients with a history of allergic reactions to medications, aciclovir should be used with caution, and the patient should be observed for signs of hypersensitivity. Overall, with appropriate dose adjustment for renal function and careful monitoring in at-risk populations, aciclovir can be used safely across many patients.
Resistance patterns
Resistance to aciclovir among herpes simplex virus and varicella-zoster virus isolates is an important clinical concern, particularly in immunocompromised patients who receive prolonged or repeated courses of therapy. The prevalence of aciclovir resistance in immunocompetent patients is low, typically below 1% for HSV and below 1% for VZV in most surveillance studies. In immunocompromised populations, however, resistance rates are higher, with estimates ranging from 3-10% for HSV and 1-5% for VZV, depending on the patient population and the duration of exposure to the drug. The mechanisms of resistance to aciclovir are well characterized and involve mutations in the viral genes encoding thymidine kinase (TK) or DNA polymerase. The most common mechanism, accounting for approximately 95% of resistant HSV isolates, is reduced or absent activity of viral TK. TK is required for the initial phosphorylation of aciclovir to its active form, and without functional TK, aciclovir cannot be activated and remains inert. Three types of TK mutations have been described: mutations that result in complete loss of TK activity (TK-negative), mutations that produce an enzyme with reduced affinity for aciclovir (TK-altered), and mutations that lead to production of a truncated, non-functional protein (TK-partial). TK-negative mutants are the most common and are highly resistant to aciclovir and related drugs that require TK activation. The second mechanism of resistance, observed in approximately 5% of resistant HSV isolates, involves mutations in the viral DNA polymerase gene. These mutations alter the structure of the DNA polymerase so that it has reduced affinity for aciclovir triphosphate, allowing viral DNA replication to proceed despite the presence of the drug. DNA polymerase mutants may exhibit cross-resistance to other antiviral agents that target the viral DNA polymerase, including ganciclovir, penciclovir, and foscarnet. The clinical significance of aciclovir resistance is most apparent in immunocompromised patients, particularly those with HIV infection, hematopoietic stem cell transplant recipients, and patients with hematologic malignancies. In these populations, resistant HSV infections can present as progressive, non-healing mucocutaneous ulcers that persist despite aciclovir therapy. These infections can be painful, debilitating, and serve as portals for secondary bacterial infections. Rarely, disseminated HSV infection with visceral involvement can occur. For VZV, aciclovir resistance is less common but has been reported, particularly in patients with HIV infection who have received prolonged aciclovir therapy for HSV prophylaxis. The management of aciclovir-resistant HSV infections involves several strategies. The first step is to confirm resistance through virologic testing, including viral culture and antiviral susceptibility testing. If resistance is confirmed or strongly suspected, alternative antiviral agents should be considered. Foscarnet, a viral DNA polymerase inhibitor that does not require TK activation, is the preferred agent for the treatment of aciclovir-resistant HSV infections. Foscarnet is administered intravenously and is associated with significant toxicity, including nephrotoxicity and electrolyte disturbances, limiting its use to patients with confirmed resistance and close monitoring. Cidofovir, another alternative, is a nucleotide analogue that also does not require viral TK for activation and has activity against aciclovir-resistant HSV isolates. It is available in both intravenous and topical formulations, though the topical formulation is not widely available. Imiquimod, an immune response modifier, has been used as an alternative or adjunctive therapy for aciclovir-resistant HSV infections in some cases. The emergence of aciclovir resistance can be minimized through appropriate use of the drug, including correct dosing, adherence to recommended treatment durations, and avoidance of unnecessary or subtherapeutic therapy. In immunocompromised patients requiring prolonged or repeated courses of aciclovir, periodic surveillance for resistant virus may be considered, though routine surveillance is not recommended. The development of new antiviral agents with activity against aciclovir-resistant viruses remains an active area of research, with agents such as pritelivir and amenamevir showing promise in clinical trials. For the general population, aciclovir resistance remains a minor clinical problem, and the drug continues to be effective for the most patients with herpes simplex and varicella-zoster infections.
Frequently asked questions
Patients commonly ask about how quickly these medications work, their safety profiles, and proper usage. Most symptoms improve within days of starting treatment, but completing the full prescribed course is essential for preventing recurrence. These medications should only be used under appropriate medical guidance. Common concerns include drug interactions, side effects, and use during pregnancy or breastfeeding. Patients are advised to discuss their specific medical history with a healthcare provider before starting any new medication and to report any persistent or unusual symptoms promptly. Generic versions of these medications offer the same therapeutic benefits as brand-name products at a lower cost.