What is PrEP? Daily pills, on-demand dosing and injections, explained

PrEP is medicine people without HIV take to prevent it: a daily pill, on-demand dosing around sex, or long-acting injections. How each works and who prescribes it.

Key points

• PrEP stands for pre-exposure prophylaxis: medicine you take before you might be exposed to HIV, so that the virus can’t take hold.
• It is for people who don’t have HIV.
• It comes as pills (daily, or on-demand around sex for some people) and as long-acting injections.
• Taken as prescribed, it cuts the risk of getting HIV from sex by about 99%.
• It does not protect against other STIs.

Who is PrEP for?

Anyone without HIV who may be exposed to it. That includes people who have sex without condoms with partners whose HIV status they don’t know, people whose partner has HIV and isn’t yet undetectable, and people who inject drugs. You don’t need to fit a category. If you think you might benefit, that is reason enough to ask.

Daily PrEP

One pill, every day. This is the most widely studied and widely available way to take PrEP, and it works for everyone: all genders and all kinds of sex.

The pills combine two HIV medicines. The most common is tenofovir disoproxil with emtricitabine, sold as Truvada and as cheaper generics. A newer pill, Descovy, is approved in the US for sexually active men but not for people at risk through receptive vaginal sex.

How long until it works? For receptive anal sex, the CDC says daily PrEP reaches maximum protection after about 7 days. For receptive vaginal or frontal sex, guidance differs: the CDC says about 21 days (and the same for injection drug use), while the NHS in the UK says 7 days. If this applies to you, ask the clinic prescribing it which to follow, and use the longer figure if in doubt.

On-demand PrEP (also called 2-1-1 or event-based dosing)

Instead of a pill every day, you take pills only around the times you have sex:

1. Two pills between 2 and 24 hours before sex
2. One pill 24 hours after that first double dose
3. One pill 24 hours after that

If you keep having sex over several days, keep taking one pill a day, and continue for two days after the last time.

Who it suits. On-demand dosing has been studied in gay and bisexual men having anal sex. The World Health Organization endorsed it for this group in 2019, and guidelines in the UK, Europe and Australia support it. UK guidance says it can be used for anal sex, and for insertive vaginal or frontal sex.

Who it doesn’t suit:

• People having receptive vaginal or frontal sex. The medicine builds up more slowly in vaginal tissue, so daily dosing is needed.
• People with hepatitis B. The same medicine treats hepatitis B, and starting and stopping it can cause problems. Get tested for hepatitis B before choosing this schedule.
• It only applies to the tenofovir disoproxil pill (Truvada and generics), not to Descovy.

Guidance differs by country. The US CDC says on-demand dosing “is not approved by the FDA and is not recommended by CDC”, while acknowledging the evidence in gay and bisexual men. If you started PrEP in one country and travel to another, a clinician there may give different advice. That is a difference in national policy, not a sign that anyone is wrong.

Injectable PrEP

Two long-acting injections now exist, given by a health worker:

• Cabotegravir (Apretude): one injection every two months.
• Lenacapavir (Yeztugo): one injection twice a year. The US approved it in June 2025, and the WHO recommended it as an additional PrEP option in July 2025. In the trials behind that decision, at least 99.9% of people who received it stayed HIV-negative.

Injections suit people who find a daily pill hard to keep up, or who don’t want pills to be found. Availability varies a great deal between countries, and they are usually more expensive than generic pills. For travellers, the important point is timing: you need to be somewhere that can give your next injection when it’s due.

What tests do you need?

• An HIV test before you start. This is required. PrEP on its own isn’t enough medicine to treat HIV, so you need to be sure you don’t already have it.
• HIV tests while you’re on it: every 3 months on pills, and at each injection visit.
• A kidney check and a hepatitis B test before starting PrEP pills, and a check for other STIs. The kidney check is repeated about once a year, more often for some people.

Side effects are generally mild. The CDC lists diarrhoea, nausea, headache, fatigue, stomach pain, and pain at the injection site, and says they usually go away over time.

Who can prescribe PrEP?

It depends on the country:

• United States. Any licensed prescriber can, with no HIV specialism needed. Pharmacists can prescribe it in some states.
• United Kingdom. From NHS sexual health clinics.
• Many other countries. Sexual health clinics, HIV clinics, community clinics and some private doctors. In some places it is sold directly by community clinics at a published price.

How to get PrEP

There are three routes, and which one fits depends on where you are and how long you’re staying:

1. A public programme. Free or subsidised in countries such as the UK, France, Australia and Brazil, usually for residents; some accept visitors. Our PrEP page lists which clinics start PrEP and what they charge.
2. A private or community clinic. Pay per visit and per bottle; prices from a few dollars a month in Thailand to well over a hundred elsewhere. Look for PrEP — start here and PrEP refill on clinic pages.
3. Online delivery. PrEP Online sells generic tenofovir/emtricitabine (the same medicine as Truvada) and tenofovir alafenamide/emtricitabine (Descovy) from about US$30 a bottle, shipped to most countries. Whichever route you use, you need an HIV test before you start and every three months while you take it, and you should check your country’s rules on importing medicines.

PrEP when you’re travelling

• Bring enough for the whole trip, plus spare, in the original packaging, with a copy of the prescription.
• A refill abroad is often possible but rarely simple. A clinic will usually want a recent HIV test. On our clinic pages, look for PrEP refill (they can supply people already taking it) as distinct from PrEP — start here.
• Time zones: for daily PrEP, a few hours either way doesn’t matter. Just keep taking one a day.
• If you’ve missed pills and had a possible exposure, that may be a situation for PEP. Ask a clinic.

Find clinics by country.

Sources

Checked 19 September 2026.

• CDC, Preventing HIV with PrEP
• CDC, Clinical guidance for PrEP
• NHS, About pre-exposure prophylaxis (PrEP)
• HIV.gov, Pre-exposure prophylaxis
• WHO, WHO recommends injectable lenacapavir for HIV prevention, July 2025
• HIV i-Base, Event-based oral dosing (2:1:1), November 2025
• aidsmap, WHO endorses event-driven PrEP for gay men, July 2019

Where To Test is a directory, not a clinical service. Rules and prices change; every figure here links to its source and the date we checked. Spotted something out of date? Use “Something wrong? Tell us” on any clinic page.