Antivirals

Key takeaways

• Antivirals treat herpes simplex and shingles (valacyclovir, acyclovir, famciclovir) or COVID-19 (molnupiravir) by blocking a specific step of viral replication, not by boosting your immune system.
• Valacyclovir and famciclovir are prodrugs that convert into acyclovir-family molecules, letting you take fewer daily doses for the same effect as acyclovir.
• The practical difference between the three herpes drugs is dosing frequency and formulation (tablet versus cream), not effectiveness against the same virus.
• The #1 safety rule: molnupiravir is not used in pregnancy, since animal studies showed a risk of fetal harm, and it is not approved for anyone under 18 because it can affect growing bone and cartilage.

How antivirals work

Antivirals block a specific step of a virus's replication cycle rather than boosting your immune defenses. Acyclovir, valacyclovir and famciclovir are converted inside infected cells into a form that stops the herpes virus from copying its DNA, so it can't spread to new cells. Molnupiravir works differently: it introduces errors into the genetic code of a replicating virus until the virus can no longer function. Starting treatment early, ideally within the first day or two of symptoms, gives the biggest reduction in severity and duration.

Choosing between valacyclovir, acyclovir, famciclovir and molnupiravir

• Valacyclovir treats herpes simplex (cold sores, genital herpes) and shingles. Your body converts it into acyclovir, which lets you take it two or three times a day instead of up to five.
• Acyclovir treats the same conditions and comes as tablets, cream and an intravenous form for severe cases, but its oral form needs dosing up to five times a day because it absorbs less efficiently than valacyclovir.
• Famciclovir also treats herpes simplex and shingles. It converts to penciclovir in the body and offers a similar dosing advantage to valacyclovir, generally two or three times daily.
• Molnupiravir is used for COVID-19 in adults at risk of severe disease, taken as a short oral course started within days of symptom onset. It is not used in pregnancy because of a risk of harm to a developing fetus, and it isn't approved for anyone under 18 because it can affect growing bone and cartilage.

Common questions

Do antivirals cure herpes or shingles?

No. They shorten outbreaks and reduce viral shedding, but the virus stays dormant in your nerve cells between episodes. Some people take a low daily dose long-term to suppress recurrences instead of treating each outbreak as it appears.

Does starting treatment late still help?

Starting within the first one to two days of symptoms gives the most benefit for herpes and shingles. Starting later still helps in some cases, especially with shingles, but the reduction in severity and duration is smaller.

Why does molnupiravir have age and pregnancy restrictions?

Animal studies showed it could harm a developing fetus, so it isn't used in pregnancy or by anyone who could become pregnant without effective contraception during and after treatment. It also isn't approved for people under 18 because of possible effects on bone and cartilage growth.

Safety essentials

• Tell your prescriber if you are pregnant, trying to conceive, or under 18 before starting molnupiravir. It is avoided in all three situations.
• Start any antiviral as soon as possible after symptoms begin. Delaying treatment reduces how much it can help.
• Stay well hydrated on acyclovir and valacyclovir, particularly at higher doses, since dehydration raises the risk of kidney effects.
• Seek urgent care for a shingles rash near the eye, spreading redness with fever, or any breathing difficulty during a COVID-19 course.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.