If your PrEP doesn't seem to be working, the usual reason is how it's being used rather than the drug itself. PrEP cuts HIV risk from sex by about 99% only when taken as prescribed and after it's had time to reach full protection CDC, PrEP. Missed doses, the wrong timing, or expecting it to block other STIs are the common gaps.
risk reduction, taken as prescribed
daily Truvada/Descovy or the Apretude injection
not other STIs or pregnancy
| Item | Value |
|---|---|
| From sex | ~99% — risk reduction, taken as prescribed |
| From injection use | ≥74% |
| Forms | pill or shot — daily Truvada/Descovy or the Apretude injection |
| Protects against | HIV only — not other STIs or pregnancy |
What PrEP is and how it works
PrEP stands for pre-exposure prophylaxis. It's medicine taken by HIV-negative people before a possible exposure to keep HIV from taking hold. It isn't a treatment for someone who already has HIV, and it isn't a cure.
The medicine keeps a protective level of antiviral drug in your bloodstream and the tissues HIV would try to infect. If the virus shows up, it can't establish a lasting infection. That protection depends on the drug already being present at the right level, so you have to take it on schedule, every time.
People often confuse PrEP with PEP. PrEP is taken on an ongoing basis before exposure. PEP is the emergency course you start after a single possible exposure and take for a short time. If you've already had a risky exposure and aren't on PrEP, talk to your provider about PEP CDC, PEP.
How well PrEP works
Taken as prescribed, PrEP reduces the risk of getting HIV from sex by about 99%. For people at risk through injection drug use, the pills lower risk by at least 74% when taken as prescribed. Those numbers describe consistent, correct use, not occasional or as-needed dosing.
PrEP isn't fully protective the day you start it. The medicine needs time to build up. Protection reaches its maximum at about 7 days for receptive anal sex, and about 21 days for receptive vaginal sex and for injection drug use. If you started PrEP this week and you're relying on it for vaginal sex, it may not be at full strength yet, and that can look like it isn't working.
The single most common mistake is treating PrEP like a morning-after pill. Taking it occasionally, or only around the times you expect sex without following the prescribed schedule, leaves the drug level too low to reliably block HIV.
How to use PrEP and who it's for
PrEP is for HIV-negative people who could be exposed to HIV through sex or injection drug use. There are a few options, and the right one depends on how you're at risk and your preferences.
| Option | Type | Who it's for |
|---|---|---|
| Truvada | Daily oral pill | People at risk through sex or injection drug use |
| Descovy | Daily oral pill | People at risk through sex only — not recommended for people assigned female at birth who are at risk through receptive vaginal sex |
| Apretude (cabotegravir) | Injection | People at risk through sex who weigh at least 77 pounds (35 kg); a choice for those who'd rather not take a daily pill |
The injectable, Apretude, is given as a shot rather than a daily pill, which can help if remembering a pill every day is the thing tripping you up. Whichever form you use, it only protects when it's in your system at the level your prescription is designed to keep there.
Starting PrEP always begins with an HIV test, because PrEP isn't the right regimen for someone who already has HIV. After that, you'll have regular check-ins while you're on it: repeat HIV testing and a quick review with your clinician. If your doses have been spotty, that visit is where to be honest about it. The fix is usually adjusting the routine rather than the drug.
Cost and how to get PrEP
You can get PrEP from a primary care clinic, a sexual-health clinic, or increasingly through telehealth services that prescribe it and arrange lab work. Cost worries many people, but assistance programs exist to cover the medication and visits for many, so don't assume the sticker price is what you'll pay. A clinic or telehealth provider can point you to the program that fits your situation.
If you're managing real side effects from the pills, those are usually mild and often settle, but they're worth raising at a visit. For what to expect and how they're handled: prep side effects.
What PrEP does NOT protect against
PrEP is HIV-specific. It does nothing to prevent other sexually transmitted infections. Chlamydia, gonorrhea, and syphilis can all still be passed even when your PrEP is working perfectly. It also doesn't prevent pregnancy.
That's why people on PrEP keep testing for other STIs on a regular schedule. If you assumed PrEP covered everything, it can seem to be failing while it does its job on HIV and an untreated gonorrhea or chlamydia goes unnoticed. Stay on top of routine screening: get tested.
How PrEP fits with the rest of prevention
PrEP works best as one layer, not the whole wall. Pairing it with the other tools closes the gaps it can't:
- Condoms add protection against the STIs and the pregnancy risk that PrEP doesn't touch.
- Regular STI testing catches the infections PrEP isn't built to stop, and many can be cured quickly once found.
- Vaccines (such as for HPV and hepatitis B) cover infections no pill or shot for HIV addresses.
- Treatment as prevention matters across the community. When people living with HIV are on effective treatment, transmission risk drops; earlier hiv treatment can help prevention explains how that works alongside PrEP.
If you've had a recent exposure and are wondering when a test will actually be accurate, timing matters as much as it does for PrEP itself; see when to test after exposure.
When to talk to a clinician
Bring it up if any of these apply: you've been missing doses or aren't sure you've reached full protection yet; you started PrEP very recently and had sex before the build-up window passed; you've had a possible HIV exposure and aren't sure whether PrEP or PEP is the right move; or you've noticed symptoms that could be another STI. None of these mean PrEP failed. Tell your provider your real routine, because that's what lets them make the plan work.