Acyclovir

Acyclovir is prescribed to treat:

• Herpes infections, chickenpox, and shingles • Various related conditions in the antivirals category • Associated symptoms and complications

It is an important medication that helps manage these conditions effectively.

Acyclovir (Zovirax) is an antiviral used for herpes simplex infections, shingles, and varicella. It was the first genuinely selective antiviral drug, and the mechanism that makes it selective is worth understanding because it explains both its remarkable safety and its narrow spectrum.

Mechanism of Action

Acyclovir is a guanosine analogue that requires three phosphorylation steps to become active. The first and rate-limiting step is performed by viral thymidine kinase, an enzyme present only in herpesvirus-infected cells. Host cellular kinases complete the remaining two steps.

This is the source of its selectivity: acyclovir is essentially inert in uninfected cells, because the first activating enzyme is not there. Only cells harbouring the virus convert it to the active triphosphate, which then inhibits viral DNA polymerase and terminates the growing DNA chain when incorporated. Concentrations of active drug inside infected cells are far higher than elsewhere in the body, which is why a drug that interferes with DNA synthesis is nonetheless well tolerated.

The same dependence explains the limits. Acyclovir works against herpes simplex 1 and 2 and varicella-zoster, with the latter requiring higher doses because it is less susceptible. It has no activity against viruses lacking thymidine kinase, and it does not eradicate latent virus residing in sensory ganglia — which is why herpes infections recur after treatment and why suppression rather than cure is the realistic goal.

Valacyclovir is a prodrug of acyclovir with far better oral bioavailability, allowing less frequent dosing. For most oral indications valacyclovir has largely replaced acyclovir on convenience grounds, while acyclovir remains standard intravenously and topically.

Clinical Use

Timing determines effectiveness more than dose does. Antivirals inhibit viral replication, so they help most when replication is still active — within 72 hours of rash onset in shingles, and at the first prodromal symptoms in recurrent herpes simplex. Started late, when lesions are already crusting, the benefit is small.

This has a practical consequence for recurrent herpes labialis or genital herpes: patients should hold a supply and start at the first tingling or burning rather than waiting for a clinician appointment, which usually arrives after the useful window has closed.

In shingles, treatment reduces the duration of the rash and acute pain and probably reduces post-herpetic neuralgia, though not as reliably as patients hope. Post-herpetic neuralgia is the complication that matters most, and antiviral treatment does not prevent it altogether — gabapentin and pregabalin are the mainstays once it develops.

Suppressive therapy is used for frequent recurrences of genital herpes and reduces both outbreaks and transmission to partners. Vaccination is the more important intervention for shingles: the recombinant zoster vaccine is highly effective and is recommended for adults from age 50, which prevents far more disease than treating episodes does. The CDC shingles resource covers vaccination and treatment.

Monitoring and Follow-Up

Short oral courses in healthy patients require no laboratory monitoring. Kidney function should be checked before high-dose or intravenous therapy and in patients with existing impairment.

Nephrotoxicity is the main safety consideration and follows a specific mechanism: acyclovir has limited solubility and can crystallise in renal tubules when concentrated. Adequate hydration substantially reduces this risk, which is why patients on higher doses are told to drink well — advice that is easy to omit and genuinely matters. Risk is highest with rapid intravenous infusion, dehydration, and existing kidney impairment.

Neurotoxicity — confusion, hallucinations, tremor, myoclonus — occurs mainly with accumulation in kidney impairment and in older patients, and is frequently misattributed to the infection or to delirium of other cause.

Dose adjustment for kidney function is essential and is among the more commonly missed adjustments, particularly when a shingles dose is prescribed to an older patient whose creatinine looks normal but whose eGFR is reduced. The MedlinePlus acyclovir entry covers prescribing detail.

Special Populations

In older adults, reduced kidney function is common and often unrecognised, making both nephrotoxicity and neurotoxicity more likely. Dose reduction by eGFR and attention to hydration are the two practical safeguards.

In pregnancy, acyclovir has substantial accumulated safety data and is used for significant herpes infections; suppressive therapy in late pregnancy reduces the need for caesarean delivery in women with genital herpes. It is compatible with breastfeeding.

In immunocompromised patients, herpes infections are more severe, more prolonged, and more likely to disseminate, and treatment is at higher doses for longer. Resistance is essentially confined to this group and arises through thymidine kinase mutations — which, by the mechanism above, also confer resistance to valacyclovir and famciclovir, so a different class such as foscarnet is required.

Topical acyclovir has limited efficacy compared with oral therapy and is generally not worth using when an oral option is available.

Chickenpox in an adult deserves separate mention, because it behaves very differently from the childhood illness. Adults have substantially higher rates of pneumonia, encephalitis, and hospitalisation, and antiviral treatment is recommended rather than optional — started within 24 hours of rash onset, which is a narrower window than for shingles. Non-immune adults, particularly those who are pregnant or immunocompromised, should also be considered for post-exposure prophylaxis after a significant exposure.

When to Contact Your Doctor

Seek urgent care for shingles involving the eye or the tip of the nose, which indicates ophthalmic involvement and threatens vision, and for shingles affecting the ear with facial weakness or hearing change. Widespread rash beyond a single dermatome, particularly in an immunocompromised patient, needs prompt assessment.

Report confusion, hallucinations, tremor, or unusual drowsiness, which may indicate accumulation. Report reduced urine output. Report severe pain persisting after the rash resolves, which is post-herpetic neuralgia and is treatable.

If you have recurrent herpes outbreaks, ask about holding a supply to start at the first symptom, and about suppressive therapy if recurrences are frequent — both are more effective than treating each episode as it arises.

To discuss shingles treatment, herpes suppression, or vaccination, contact us or schedule a visit.