Description

Zovirax (acyclovir)

Most people buying Zovirax are dealing with one of three things: a cold sore that appeared overnight, a shingles diagnosis, or recurrent genital herpes they’re trying to manage. The drug is the same active ingredient — acyclovir — across all three situations. What changes is the dose, the timing, and what you can realistically expect it to do.

Available in 200 mg, 400 mg, and 800 mg tablets.

Cold sores

Oral acyclovir reduces the duration and severity of cold sore outbreaks, but only if started early. The relevant window is the prodrome — the tingling, itching, or burning that precedes the visible blister. Once the lesion is fully formed, the antiviral effect is marginal. Dose: 200 mg five times daily for five days. If you’ve missed the early window, topical cream still provides some benefit but the oral course is more effective when started promptly.

Shingles

Shingles requires the higher 800 mg dose, five times daily, for 7 to 10 days. Treatment must begin within 72 hours of the rash appearing — after that point, the evidence for meaningful benefit weakens considerably. Acyclovir reduces the severity and duration of the acute outbreak and lowers, though doesn’t eliminate, the risk of post-herpetic neuralgia — the nerve pain that can persist for months after the rash resolves.

Genital herpes

For a first episode: 200 mg five times daily, or 400 mg three times daily, for five days. First episodes tend to be more severe than recurrences, and treatment started early reduces that severity. For recurrent outbreaks in people who get them frequently (more than six per year), continuous suppressive therapy at 400 mg twice daily reduces outbreak frequency and also lowers the risk of transmission to partners. Suppressive therapy is typically reviewed annually.

Bioavailability and what that means practically

Oral acyclovir has relatively low bioavailability — only about 15 to 30% of the dose reaches systemic circulation. This is a known limitation of the drug, which is why doses need to be taken multiple times daily and why more severe or systemic infections (disseminated herpes, herpes encephalitis, herpes in immunocompromised patients) require intravenous rather than oral treatment.

Side effects

Nausea, abdominal discomfort, headache, fatigue, and rash are the most commonly reported. At standard oral doses in people with normal kidney function, neurological effects are rare. In elderly patients or those with kidney impairment, confusion or dizziness are more likely and dose adjustment is needed.

Renal function and hydration

Acyclovir clears through the kidneys. Reduced kidney function leads to drug accumulation. Anyone with known renal impairment should have the dose adjusted before starting. Staying well-hydrated during a course of high-dose oral treatment (the 800 mg shingles regimen particularly) reduces the risk of crystalluria.

Important to know

Acyclovir suppresses herpes virus activity but doesn’t clear the virus from the body — the virus remains latent in nerve ganglia. Even on suppressive therapy, the risk of transmission to a sexual partner is reduced but not eliminated. Condoms remain the most reliable protection against transmission.