“How long does PrEP take to work?” sounds like it should have one answer. It does not. The answer depends on the type of sex (or drug use) you want protection for, and on which country’s guidance you follow. US and UK advice differ more than most people realise.
This guide covers PrEP pills: emtricitabine with tenofovir disoproxil fumarate (Truvada and its generics) and emtricitabine with tenofovir alafenamide (Descovy and its generics). For injections, see our guide to the PrEP shot.
The US answer: 7 days or 21 days of daily pills
The CDC says that with daily PrEP pills1:
- for receptive anal sex (bottoming), maximum protection is reached after about 7 days of daily use
- for receptive vaginal sex and injecting drug use, maximum protection is reached after about 21 days of daily use
- for insertive sex (topping, or the penis-in-vagina partner), there are no data on the time needed
The CDC also describes the on-demand “2-1-1” schedule: 2 pills 2 to 24 hours before sex, 1 pill 24 hours after the first dose, and 1 more pill 24 hours later. It says there is evidence this works for gay and bisexual men having anal sex, but not enough evidence for vaginal sex or injecting drug use, and that the schedule is not FDA-approved1.
The UK answer: a double-dose start for everyone
The 2025 UK guidelines from the British Association for Sexual Health and HIV (BASHH) and the British HIV Association (BHIVA) take a newer approach. Daily PrEP can be used by anyone, for any type of exposure. And PrEP can be started by anyone with a double dose (two pills) 2 to 24 hours before sex, with the stopping rule depending on the type of exposure2:
| Type of exposure | Start | Keep going until |
|---|---|---|
| Receptive anal sex | Two pills 2 to 24 hours before | One pill a day for 2 days after the last sex (the 2:1:1 pattern) |
| Insertive anal or vaginal sex | Two pills 2 to 24 hours before | One pill a day for 2 days after the last sex |
| Receptive vaginal sex | Two pills 2 to 24 hours before | One pill a day for 7 days after the last sex (the 2:7 pattern) |
| Injecting drug use | Two pills 2 to 24 hours before | One pill a day for 7 days after the last risk |
The UK guideline stresses that for receptive vaginal sex and injecting, taking fewer than seven daily doses after the double-dose start is likely to give less protection2. It also notes that taking the first dose with food may reduce stomach upset2.
Which should you follow? The one your prescriber gives you. If you are in the US, your clinic will usually follow CDC advice. If you are in the UK, the double-dose start is standard. Our guide to daily vs on-demand PrEP explains the two ways of taking PrEP in more detail.
Truvada vs Descovy: who can use which
Both pills are used for PrEP, but Descovy is not approved for people at risk through receptive vaginal sex3. If that is your main risk, Truvada or a generic version of it is the usual choice. Our guide to Truvada vs Descovy explains the difference between the two forms of tenofovir.
Missed a PrEP pill?
On daily PrEP: one missed pill is not a reason to panic. The UK guideline says that once daily dosing is established (at least four doses in the week before an exposure), taking four doses a week in the following weeks is likely to give good protection for all types of exposure2. Take your next pill as normal and get back to your routine. Daily is still the target.
On 2:1:1 and late with the after-sex dose: the UK guideline says the first after-sex dose can still be taken up to 48 hours after sex, as long as you took at least one pill before sex. Take the second one 24 hours after that2.
More than 48 hours late, or on 2:7: take a dose and carry on daily while you get advice from a sexual health clinic2.
Several pills missed, then condomless sex? You may need PEP. The UK guideline advises restarting with a double dose as soon as possible, preferably within 24 hours and no later than 72 hours, and contacting a clinic about PEP2. BHIVA has published a short checklist of when an oral PrEP user does not need PEP, based on how recently you took a pill and whether you took a double dose within 24 hours afterwards4. If in doubt, ask a clinic. See our guide to PEP after a possible exposure.
Took two pills by mistake?
A double dose is how PrEP is started in the UK schedule2. If you have taken more than you were prescribed, follow the leaflet: the Truvada patient information says that if you take too much, you should call your healthcare provider or go to the nearest emergency room5. Then return to your normal schedule.
Stopping PrEP safely
Do not just stop the day after your last sex. In the UK guideline, you keep taking one pill a day for 2 days after your last anal sex or insertive vaginal sex, and for 7 days after your last receptive vaginal sex or injecting risk2. The CDC advises talking to your provider about how to stop safely and about other prevention options1.
If you have hepatitis B, never stop PrEP without your doctor. Both PrEP pills are also active against hepatitis B, and stopping them can cause a serious flare of liver inflammation. This is a boxed warning on the Truvada label5. That is why a hepatitis B test is part of starting PrEP, and why people without immunity are offered the hepatitis B vaccine2. Our guide to PrEP and hepatitis B explains what this means for you, and our hepatitis B treatment guide covers treatment.
Restarting after a break: test first
Before you restart, get an HIV test. PrEP pills on their own are not enough to treat HIV, and taking them while an infection goes undiagnosed can lead to drug resistance. The Truvada label carries a boxed warning about exactly this5. The UK guideline recommends a laboratory HIV test (which can be a self-sample kit sent to a lab) before starting oral PrEP, rather than a home self-test, and repeat tests every 3 months for people with new or multiple partners2.
If you had a possible exposure during your break, the PEP advice above applies instead.
If you buy PrEP online
Many people source generic PrEP online. The UK guideline asks clinics to give people who do this the same full support, including kidney monitoring, and recommends the same HIV testing schedule2. Whatever route you use, keep up the tests. Our PrEP guide explains prescriptions, testing and import rules, and covers generic PrEP such as Tenvir-EM (the same ingredients as Truvada) and Tafmune-EM (the same ingredients as Descovy).
Frequently asked questions
How long after starting PrEP is it effective?
In the US, the CDC says about 7 days of daily pills for receptive anal sex and about 21 days for receptive vaginal sex and injecting drug use. The 2025 UK guidelines allow anyone to start with a double dose 2 to 24 hours before sex. Follow the schedule your prescriber gives you.
What if I miss a PrEP pill?
On daily PrEP, take your next pill as normal; UK guidance says four doses a week is likely to keep good protection once daily dosing is established. If you have missed several pills and had condomless sex, contact a clinic about PEP within 72 hours.
How long does PrEP keep protecting you after your last pill?
That is why the stopping rules exist. UK guidance says to keep taking a pill daily for 2 days after the last anal or insertive sex, and 7 days after the last receptive vaginal sex or injecting risk. Stop earlier and your last exposure may not be covered.
Can I stop PrEP any time?
You can stop, but follow the stopping rule for your type of exposure, and never stop without medical advice if you have hepatitis B. Before restarting later, get an HIV test.
Should I take PrEP with food?
PrEP pills can be taken with or without food. The UK guideline notes that taking the first dose with food may reduce stomach upset.
Does PrEP work straight away for women?
CDC advice is about 21 days of daily pills for receptive vaginal sex. The UK 2025 guideline allows a double-dose start followed by daily pills, continued for 7 days after the last sex. Descovy is not approved for people at risk through receptive vaginal sex.
Sources & references
- Centers for Disease Control and Prevention. Preventing HIV with PrEP.
- British Association for Sexual Health and HIV and British HIV Association. BASHH/BHIVA guidelines on the use of HIV pre-exposure prophylaxis (PrEP) 2025.
- HIV.gov. Pre-Exposure Prophylaxis (PrEP). Updated 9 October 2026.
- British HIV Association. Clarification of an oral PrEP user who does not need PEP. 2026.
- US National Library of Medicine, DailyMed. Truvada prescribing information and patient information.
Related reading
- How to get PrEP
- PrEP with alcohol, drugs and hormones
- PrEP vs PEP
- PrEP: daily vs on-demand
- Generic Truvada and generic Descovy
- All HIV and PrEP medicines
This content is for informational purposes only and is not medical advice. PrEP schedules differ between countries and people; follow the plan agreed with your prescriber, and test for HIV regularly. Used as prescribed by a licensed medical professional.
David Kelly is a licensed clinical pharmacist with 13 years of experience. He holds a B.Pharm from Delhi University. At OnlineMeds, he reviews medicine and health content to help ensure it is accurate, evidence-based, and easy to understand.