Acyclovir has no single moment after which you stop being contagious. It lowers the amount of virus you release but doesn’t switch it off. The drug blocks the herpes virus from copying itself while leaving your own cells largely alone.1PubMed. Mechanism of action and selectivity of acyclovir Herpes can also spread when you feel completely fine, so a healed outbreak is not the same as zero risk.2PubMed Central. Herpes simplex virus-2 transmission probability estimates based on quantity of viral shedding The useful answer comes in two parts: how fast acyclovir shortens an outbreak, and how much daily use protects a partner between outbreaks.
How Fast Acyclovir Shortens a Contagious Outbreak
You can pass herpes on while live virus leaves your skin, which is called viral shedding. It happens from sores and sometimes from skin that looks normal. In repeat genital outbreaks, treated people shed virus for about 1 day on average, compared with about 2 days without treatment.3PubMed. A trial of topical acyclovir in genital herpes simplex virus infections Taken by mouth, acyclovir stopped shedding in repeat outbreaks a little over twice as fast as no treatment.4PubMed. A randomized, placebo-controlled comparison of oral valacyclovir and acyclovir in immunocompetent patients with recurrent genital herpes infections Pills also speed healing and prevent new sores more than cream does.5PubMed. Treatment of primary first-episode genital herpes simplex virus infections with acyclovir: results of topical, intravenous and oral therapy
Pills beat cream for protecting a partner because cream only reaches the skin it touches. Pills act throughout the body, including the nerve endings where the virus reactivates, so a surface cream can miss nearby skin that is shedding but looks normal. That is why a prescription pill is the usual choice for genital herpes.
The first outbreak and later ones both respond. In a first genital outbreak, acyclovir 200 mg five times a day for 5 days shortened shedding, pain, itching, and healing time.6Lancet. Efficacy of oral acyclovir in the treatment of initial and recurrent genital herpes In later outbreaks the same course shortened shedding and healing and reduced new sores.7Lancet. Efficacy of oral acyclovir in the treatment of initial and recurrent genital herpes
Timing matters, because acyclovir works by stopping new virus from being made. It helps most before lots has built up, so starting at the first tingle or itch gives the shortest outbreak. People who keep a prescription on hand can start within hours.
The pills do not heal sores overnight. New blisters stop appearing sooner, pain eases, and existing sores crust over faster, but they still need days to heal. Feeling better partway through doesn’t mean you can stop early or resume contact, so finish the full prescribed course.
The risk stays high from the first tingle until sores are fully healed.8Pharmica. The five stages of a genital herpes outbreak The 1-to-2-day figures are averages, and some people shed longer. Tests that look for live virus can miss smaller amounts that may still spread, and healing skin can release virus even after a blister has crusted. Visible healing is a more reliable marker than a day count, so wait until all sores have healed and new skin has formed before sexual or skin contact.
Why You Can Still Spread Herpes Without Sores
Most sexual transmission of genital herpes happens while the person has no symptoms.9PubMed Central. Herpes simplex virus-2 transmission probability estimates based on quantity of viral shedding Many shedding episodes produce sores too small to notice.10PubMed Central. Herpes simplex virus-2 transmission probability estimates based on quantity of viral shedding In women with genital herpes type 2 and no treatment, sensitive tests found virus on about 28% of days.11PubMed. Frequent genital herpes simplex virus 2 shedding in immunocompetent women. Effect of acyclovir treatment. Even the lowest-risk stage between outbreaks still carries some risk.12Pharmica. The five stages of a genital herpes outbreak
Many people with herpes never know they shed, and a partner can catch it from someone who has never had a noticeable outbreak. Silent shedding can happen on skin that looks entirely normal, including areas well away from where past sores appeared. Since you can’t see or feel most shedding, steady habits protect you better than checking your skin before contact.
Cold sores work the same way. The cold sore virus is shed both during outbreaks and when there are no symptoms.13Oxford Academic (J Antimicrob Chemother). The many challenges of facial herpes simplex virus infection Kissing during a cold sore carries the most risk, but symptom-free spread is possible. Pills or creams both help clear an outbreak, and daily pills can be used for frequent ones.14Oxford Academic (J Antimicrob Chemother). The many challenges of facial herpes simplex virus infection
Shedding often begins with the tingling, before anything shows. Avoid kissing babies and sharing lip balm or cups during and around an outbreak.
How Much Daily Acyclovir Lowers the Risk to a Partner
Taking an antiviral every day to prevent outbreaks is called suppressive therapy. In women taking daily acyclovir, symptom-free shedding fell from about 7 in 100 days to fewer than 1 in 100, a 94% drop.15PubMed. Suppression of subclinical shedding of herpes simplex virus type 2 with acyclovir Both acyclovir and valacyclovir, a related pill the body converts into acyclovir, greatly lowered shedding compared with no treatment.16PubMed. Valacyclovir and acyclovir for suppression of shedding of herpes simplex virus in the genital tract Still, about 7 in 48 people on daily acyclovir still shed virus without symptoms.17PubMed. Asymptomatic herpes simplex virus shedding from the genital tract whilst on suppressive doses of oral acyclovir
A 94% drop is a large fall in the share of days with virus on the skin, not a guarantee that any given day is virus-free. It describes an average, not your personal level.
In couples where one partner had genital herpes, daily valacyclovir cut the partner’s chance of getting infected from 3.6% to 1.9% over about 8 months.18PubMed. Once-daily valacyclovir to reduce the risk of transmission of genital herpes The chance of the partner getting symptomatic herpes fell by about three-quarters.19PubMed. Once-daily valacyclovir to reduce the risk of transmission of genital herpes That fits with what was found on the skin: virus turned up on about 3 in 100 days with treatment versus 11 in 100 without.20PubMed. Once-daily valacyclovir to reduce the risk of transmission of genital herpes
Those percentages describe the chance of infection over about 8 months in couples, not a per-encounter risk. Condoms were paired with the medicine in those couples, so the pill is best seen as one layer of protection.
Steady protection relies on taking doses every day, not occasionally. When you first start daily pills, ask your doctor whether to use condoms more strictly in the first weeks.
For daily use, valacyclovir (Valtrex) is the pill tested in the main partner-protection study. It needs fewer doses a day than acyclovir, which makes it easier to stay protected, and the two cut shedding by similar amounts.
Daily pills are usually offered to people with frequent outbreaks or those with a partner who does not have herpes. It is a long-term commitment, so it is worth discussing how long to continue and whether to review it periodically.
Why Missing Doses Leaves Gaps in Protection
Only about 15–30% of a swallowed acyclovir dose reaches the blood.21Klinická farmakologie a farmacie. The importance of therapeutic drug monitoring in dosage optimization of acyclovir Blood levels peak 1 to 3 hours after a dose and then fall.22PubMed Central. Acyclovir Levels in Serum and Cerebrospinal Fluid after Oral Administration of Valacyclovir In modelling, drug levels were too low to block the virus for about 42% of each dosing cycle.23PubMed Central. Rapid viral expansion and short drug half-life explain the incomplete effectiveness of current herpes simplex virus 2-directed antiviral agents Herpes multiplies fast, so even short gaps can let an outbreak take hold.24PubMed Central. Rapid viral expansion and short drug half-life explain the incomplete effectiveness of current herpes simplex virus 2-directed antiviral agents
Because so little of each dose gets into the blood, acyclovir is prescribed several times a day, and even spacing matters. Taking two doses together to catch up doesn’t help, since the body absorbs only a limited share of what you swallow.
Phone reminders help. If you miss one, take it when you remember unless the next is near. If multiple daily doses are hard to keep up, ask about valacyclovir.
Low or irregular doses and long use are risk factors for the virus becoming resistant to the drug. Resistance mainly affects people with weakened immunity, and it is rare for most people. If your sores don’t improve on treatment, tell your doctor.
Other Ways to Protect a Partner
Daily antivirals, condoms, avoiding sex during outbreaks, and telling partners each reduce transmission.25PubMed. Herpes simplex virus (update on dermatologic aspects of sexually transmitted disease and human immunodeficiency virus) Condoms cover only part of the skin that can shed virus, so combining daily pills with condoms gives more protection than either alone. Condoms reduce risk only when used for every act of sex, from start to finish.
Avoiding contact during outbreaks means stopping sex and skin contact from the first tingle, burning, or itch, since shedding can start before any sore shows. Oral sex counts, because herpes passes between mouth and genitals, and contact short of intercourse can still pass the virus. Resume only once sores have healed completely and the skin looks normal, not just when they stop hurting.
Telling a partner lets them make choices about condoms and timing. A partner can also ask a doctor about testing to learn their own status.
Shingles, Pregnancy, and Weakened Immunity
In shingles, acyclovir cut the time the rash shed virus from about 5 days to 2.26PubMed. Acyclovir therapy for acute herpes zoster Shingles comes from the chickenpox virus, a different virus from the one behind cold sores and genital herpes. The rash can give chickenpox, not shingles, to people who never had it or the vaccine, until the blisters crust. Keep the rash covered, wash your hands, and avoid pregnant women and newborns until it crusts. Acyclovir works best when started early in the rash.
Acyclovir from 36 weeks of pregnancy lowers outbreaks and virus at delivery and reduces cesarean births for herpes.27PubMed. Acyclovir prophylaxis to prevent herpes simplex virus recurrence at delivery: A systematic review The aim is to keep the virus quiet around delivery, when passing it to the baby is most likely. It reduces risk rather than removing it: some women on it still had virus detected near delivery,28PubMed. A double-blind, randomized, placebo-controlled trial of acyclovir in late pregnancy for the reduction of herpes simplex virus shedding and cesarean delivery so doctors still check for symptoms when labor begins. It also cannot protect babies of women who don’t know they carry herpes.29PubMed. Acyclovir prophylaxis in late pregnancy prevents recurrent genital herpes and viral shedding
If you are pregnant, tell your obstetrician about any herpes history, even old. Ask whether to start acyclovir at 36 weeks, and report any tingling or sores near your due date. Newborns are especially vulnerable to herpes, so don’t kiss a baby when you have a cold sore, including during tingling, and wash your hands before touching the baby during an outbreak.
In people with weakened immune systems, about 4–7% of herpes infections resist acyclovir.30PubMed Central. Herpes Simplex Virus and Varicella Zoster Virus Infections in Cancer Patients Contagious periods may last longer in those cases, and a doctor may need to change treatment.