Acyclovir

Acyclovir is a targeted antiviral that works specifically inside cells infected by herpes viruses, leaving your healthy cells largely unaffected. Here is how:

- Selective activation: Acyclovir is inactive when you take it. It only becomes active inside cells that are infected with herpes virus. The virus itself produces an enzyme (thymidine kinase) that converts acyclovir into its active form. - Stops viral replication: Once activated, acyclovir tricks the virus's DNA-copying machinery into incorporating it into new viral DNA. This causes the DNA chain to terminate — the virus cannot finish copying itself. - Spares healthy cells: Because the activation step requires a viral enzyme, acyclovir has minimal effect on uninfected cells, which is why it has relatively few side effects.

Acyclovir does not cure herpes — the virus remains dormant in nerve cells. But it can shorten outbreaks, reduce their severity, and when taken daily (suppressive therapy), significantly reduce the frequency of recurrences and the risk of transmission.

Genital herpes (first episode): - Days 1-2: Symptoms may still worsen initially, but acyclovir is already working to limit viral replication. - Days 2-5: Pain and new lesion formation decrease. Existing lesions begin healing. - Days 5-10: Significant healing; pain markedly reduced. Starting treatment within 72 hours of symptom onset provides the best outcomes.

Genital herpes (recurrence): - Treatment is most effective when started at the first sign of a recurrence (tingling, itching). Starting within 24 hours can shorten the episode by 1-2 days.

Shingles (herpes zoster): - Days 1-3: New lesion formation should slow if started within 72 hours of rash onset. - Days 3-7: Pain may begin to improve. Existing lesions crust over. - Weeks 2-4: Rash resolves. Early treatment reduces the risk of postherpetic neuralgia (persistent nerve pain).

Suppressive therapy: - Months 1-12: Daily acyclovir reduces genital herpes recurrences by ~80%. It also reduces asymptomatic viral shedding by ~95%, lowering transmission risk to partners.

Food: Acyclovir can be taken with or without food. Food does not significantly affect absorption ^[1]. Taking it with food may help if nausea occurs.

Water/Hydration: Adequate hydration is important — especially with higher doses or IV formulation — to prevent crystallization in renal tubules, which can cause kidney injury ^[1]. Drink plenty of water throughout the day.

Alcohol: No direct pharmacokinetic interaction, but alcohol can impair immune function and may potentially worsen herpes outbreaks.

Genital herpes — first episode [1]: - 400 mg three times daily for 7-10 days - Alternative: 200 mg five times daily for 7-10 days

Genital herpes — recurrent episodes [1]: - 400 mg three times daily for 5 days - Alternative: 800 mg twice daily for 5 days - Start at first sign of recurrence (prodromal tingling)

Genital herpes — suppressive therapy [1]: - 400 mg twice daily (most common regimen) - Reduces recurrences by ~80% and asymptomatic shedding by ~95% - Reassess need annually

Herpes zoster (shingles) [1]: - 800 mg five times daily for 7-10 days - Start within 72 hours of rash onset for best results - Note: Valacyclovir (500-1000 mg three times daily) is often preferred for shingles due to simpler dosing and better bioavailability

Cold sores (herpes labialis): - 400 mg five times daily for 5 days

Renal dose adjustments required for CrCl <50 mL/min [1] - Available as 200 mg, 400 mg, and 800 mg tablets/capsules

Generic acyclovir is very affordable and widely available — it is one of the least expensive antiviral medications ^{[6, 7]}.

Pricing comparison: - Generic acyclovir 400 mg: $5-20/month (suppressive therapy) - Generic acyclovir 800 mg: $10-30/month (shingles treatment) - Brand Zovirax oral: rarely prescribed due to cost - Generic acyclovir 5% cream: $15-40/tube - Brand Zovirax cream: $600+/tube

Many pharmacy discount programs offer generic acyclovir tablets for $4-10 for a 30-day supply.

Pregnancy: Acyclovir has been used extensively in pregnancy and is generally considered safe ^{[1, 5]}.

- Large pregnancy registries (>1,000 exposures in first trimester) have shown no increased risk of birth defects compared to the general population ^[5] - The CDC and ACOG recommend acyclovir or valacyclovir for treatment of genital herpes in pregnancy - Suppressive therapy (400 mg twice or three times daily) is recommended starting at 36 weeks gestation to reduce the risk of active lesions at delivery, which would necessitate cesarean section ^[5]

Breastfeeding: Acyclovir is excreted in breast milk but at low concentrations. The estimated infant dose is <1% of the therapeutic infant dose. Acyclovir is considered compatible with breastfeeding by the AAP and WHO ^[5].

The Zovirax brand name is still available but is rarely prescribed because generic acyclovir is widely available and dramatically cheaper. The oral generics are AB-rated by the FDA and clinically interchangeable. One exception: the topical cream market has a large price gap between generic and brand. Also note that valacyclovir (the prodrug of acyclovir) offers better bioavailability and more convenient dosing for some indications — discuss with your doctor if the five-times-daily dosing of acyclovir is burdensome.