Aciclovir

Aciclovir is an antiviral tablet used to treat infections caused by the herpes simplex virus (HSV) — the same virus family responsible for cold sores around the mouth and lesions in the genital area. It works by blocking the virus's ability to copy its own DNA. Cleverly, aciclovir only activates inside cells that are already infected by the herpes virus, because it relies on a viral enzyme (thymidine kinase) to switch it on. That selectivity is one of the reasons it's so well tolerated — healthy, uninfected cells largely ignore it. Once activated, aciclovir slots itself into the viral DNA chain and stops replication, which means the outbreak runs out of fuel sooner and the body's immune system can catch up.

No — and it's important to be honest about that, because online searches often hint otherwise. Once you've caught the herpes simplex virus, it travels along nerve fibres to a cluster of nerve cells (a ganglion) near the base of the spine, where it goes into a kind of dormant state. From there it occasionally reactivates, travels back down the nerve, and produces an outbreak. Aciclovir is highly effective at controlling those active phases — it shortens outbreaks, reduces their severity, and in daily use cuts down how often they happen — but it doesn't reach the dormant virus inside the nerve cells, and no current medicine does. The encouraging news is that, for most people, outbreaks become less frequent and less severe over the years even without treatment, and the virus poses no long-term threat to general health.

These are the two main ways aciclovir is used in genital herpes, and the choice depends on how often you have outbreaks and how much they affect your life. Episodic treatment means taking a short course of tablets at the first sign of an outbreak — usually at higher doses, for two to five days — to cut the outbreak short. It works best for people who have only occasional recurrences. Suppressive treatment means taking aciclovir every day at a lower dose to stop outbreaks happening in the first place. It's typically considered for people who have frequent outbreaks (often defined as six or more a year), or fewer outbreaks that are severe, painful, or significantly affecting mood, relationships, or daily life. Suppressive therapy can reduce recurrences by around 70 to 80 per cent and is reviewed periodically — often after a year — to see whether continuing makes sense.

As early as you possibly can. The earlier you start, the more dramatically aciclovir shortens an outbreak, because the medicine works by blocking new viral copies — and most of those copies are made in the first 24 to 48 hours. Many people learn to recognise the very early warning signs, often called the prodrome: tingling, itching, burning, or a vague aching feeling in the area where the outbreak usually appears, sometimes a day or two before any visible lesions. Starting aciclovir at that prodromal stage can occasionally stop the outbreak from fully developing at all. If lesions are already established, treatment is still worthwhile but the benefit is more modest. This is why people on episodic regimens are often given a "standby" prescription to keep at home so they can start treatment immediately rather than waiting for a GP appointment.

Doses vary depending on whether the outbreak is your first ever or a recurrence. A first episode is usually treated for five days, often with 400 mg three times a day or 200 mg five times a day. Recurrent episodes are typically treated with shorter, sharper courses — for example 800 mg three times a day for two days, which works just as well as longer courses for most people. Suppressive therapy is usually 400 mg twice daily, sometimes adjusted depending on response and tolerability. Always follow the exact dose your prescriber gives you, because the right regimen depends on whether you have any kidney issues, what your weight is, and whether you have an immune-suppressing condition.

Many people feel an improvement within 24 to 48 hours of starting aciclovir, particularly if they began treatment at the prodromal stage. Pain and tingling tend to ease first; the visible blisters and ulcers may take a few days longer to crust over and heal. A typical recurrent outbreak might last around 7 to 10 days untreated, and aciclovir often shortens that by roughly two to three days, sometimes more if treatment is started very early.

Aciclovir is generally very well tolerated, which is one of the reasons it's used so widely. The most commonly reported effects are mild nausea, headache, tiredness, dizziness, and abdominal discomfort. Some people develop a rash. More rarely, aciclovir can affect the kidneys, particularly at very high doses, in older patients, or in people who become dehydrated — which is why drinking plenty of water during a course of treatment is sensible. Even rarer but worth knowing about are confusion, hallucinations, or unusual drowsiness, which are mostly seen in people with significant kidney problems. Any sudden new symptoms while on aciclovir, particularly a rash with fever or a marked drop in urine output, should always be assessed by a clinician.

Not necessarily, and this is a common worry. People on episodic treatment only take tablets during outbreaks, so there's no ongoing daily commitment. People on suppressive therapy usually take aciclovir for several months to a year, and then stop for a "trial off" period — partly because outbreak frequency naturally tends to drop over time, and partly to confirm that suppression is still needed. Many people find that after a year or two of suppression, recurrences are far less frequent and they're happy to switch to episodic treatment, or even to no treatment at all. This is a conversation to have with your prescriber rather than something to decide alone, because stopping suppressive therapy abruptly can lead to a brief uptick in outbreaks while the body adjusts.

It significantly reduces the risk, but doesn't eliminate it — and this is one of the most important honesty points in this whole topic. Herpes can be transmitted not only when visible lesions are present but also during periods of asymptomatic shedding, where the virus is on the skin without any symptoms at all. Suppressive aciclovir reduces both the frequency of visible outbreaks and the amount of asymptomatic shedding, and studies have shown around a 50 per cent reduction in transmission to a previously uninfected partner. That's a meaningful drop, but it isn't zero. Combining suppressive therapy with consistent condom use, avoiding sex during prodromal symptoms or outbreaks, and being open with sexual partners about your status gives the best overall protection. Disclosure is often the hardest part for patients, but partners almost always cope better with honest information than with secrecy and surprise.

Yes, in most cases. Aciclovir has been studied extensively in pregnancy and is generally considered safe, with reassuring data going back several decades. For pregnant women with genital herpes, UK BASHH guidance often recommends suppressive aciclovir from around 36 weeks of pregnancy onwards to reduce the chance of an outbreak (and therefore lesions on the genital tract) at the time of delivery — because the main concern in pregnancy is neonatal herpes, a rare but serious infection that can occur if a baby is exposed to active herpes during birth. If you have a first episode of genital herpes during pregnancy, particularly close to delivery, you'll be referred urgently to obstetrics, because the management is different. Aciclovir is also compatible with breastfeeding; only very small amounts pass into breast milk.

Aciclovir has relatively few important drug interactions, which is another of its strengths. The main one is probenecid (a gout medication), which slows aciclovir's clearance from the body and can increase its levels — usually managed by dose adjustment rather than avoidance. Caution is also sensible with other medicines that can affect the kidneys, such as long-term high-dose NSAIDs (ibuprofen, naproxen) or certain antibiotics, particularly if you already have any kidney impairment. Always tell prescribers about everything you're taking, including herbal remedies and over-the-counter painkillers, so any interactions can be considered properly.

Take it as soon as you remember, unless it's nearly time for the next dose, in which case skip the missed one and carry on as normal — never double up. If you're on a short episodic course and miss a dose, the course is unlikely to fail because of one slip; just resume the schedule. If you're on suppressive therapy and find yourself missing doses regularly, that's worth flagging at your next review, because reliable daily dosing is what keeps the suppression working.

These three medicines are close cousins. Valaciclovir is essentially aciclovir in a pre-packaged form (a prodrug) that the body converts into aciclovir after absorption — it gives higher and more sustained blood levels, which means fewer tablets per day and often more convenient dosing. Famciclovir works in a similar way and is also dosed less frequently than aciclovir. Clinically, all three are highly effective for genital herpes, and the choice often comes down to dosing convenience, cost, and prescriber preference. Aciclovir remains widely used because it's well established, affordable, and has the most extensive long-term safety data — particularly in pregnancy. If swallowing tablets four or five times a day during an outbreak is impractical for you, mention it to your prescriber, because valaciclovir's twice-daily regimen may suit your routine better.

A few situations call for in-person assessment rather than continuing alone. A first-ever episode should always be confirmed by a clinician, ideally with a swab to identify the virus type, because the diagnosis carries long-term implications and other conditions can mimic it. Outbreaks that are spreading widely, becoming unusually painful, accompanied by fever, difficulty passing urine, or affecting your ability to walk all warrant urgent review — particularly the last two, which can be a sign of a more severe first episode requiring stronger treatment. If you're pregnant and develop symptoms, contact your midwife or GP promptly. And if outbreaks become more frequent than once every couple of months despite treatment, or are taking a real toll on mood or relationships, a discussion about long-term suppressive therapy is well worth having — it's a treatable problem, and there's no need to push through quietly.