PrEP (pre-exposure prophylaxis) is medicine that people without HIV take to stop HIV from taking hold if they’re exposed. It’s one of the most effective prevention tools there is, but how well it works for you depends on taking it as prescribed.

How well PrEP works

CDC estimates that PrEP lowers the risk of getting HIV from sex by about 99% for men who have sex with men, when taken daily. For heterosexual men and women, the CDC’s figure is lower, about 90%, based on recent use. PrEP lowers the risk from injection drug use by at least 74%. The calculator uses the first figure for anal and oral sex and the second for vaginal sex.

To see what that means, take receptive anal sex with a partner who has HIV and isn’t on treatment, the highest-risk type of sex. Without PrEP, the estimated risk is 1.4% (about 1 in 72) per act. With PrEP taken consistently, it falls to 0.014% (about 1 in 7,200). For a woman having vaginal sex with a man who has HIV, it falls from 0.08% (about 1 in 1,200) to 0.008% (about 1 in 12,000). That is a smaller drop, because PrEP’s estimated effect for vaginal sex is lower.

Why missed doses matter

PrEP only works if there’s enough medicine in your body when an exposure happens. The CDC puts it plainly: you must take PrEP as prescribed for it to work.

The calculator uses three levels for the same act of receptive anal sex with a partner who has HIV:

  • Taken consistently: 0.014% (about 1 in 7,200)
  • Mostly consistent, with some missed doses: 0.14% (about 1 in 720)
  • Taken inconsistently: 0.69% (about 1 in 140)

For receptive vaginal sex, the same three levels give 0.008% (about 1 in 12,000), 0.02% (about 1 in 5,000) and 0.048% (about 1 in 2,100).

These middle and lower levels are the calculator’s working assumptions, not exact measurements. The direction is clear, though: the more doses you miss, the less protected you are.

If you’ve missed several doses and had a possible exposure, contact a clinician straight away. They can tell you whether you need PEP (post-exposure prophylaxis), which must start within 72 hours.

When protection starts

Protection isn’t immediate when you first start. The CDC says PrEP pills reach maximum protection after about 7 days of daily use for receptive anal sex, and about 21 days for receptive vaginal sex and injection drug use. With PrEP shots, the timing depends on the type of injection.

Before you start, you need an HIV test to make sure you don’t already have HIV.

Had a possible exposure before PrEP was fully working, or after stopping it? Ask a clinician about PEP. It must start within 72 hours, ideally within 24. See how to get PEP.

PrEP options

  • Daily pills. The most widely used option.
  • Event-driven pills, taken around the time of sex. This is an option for some men who have sex with men. Your clinician will explain the exact schedule.
  • Long-acting injections, given by a clinician at set intervals.

If you’ve just finished PEP and expect more exposures, CDC guidelines support moving straight onto PrEP without a gap, with an HIV test at the end of the PEP course.

Stopping and restarting

The CDC advises talking to your health care provider if you’re thinking about stopping PrEP, so you can plan it safely and keep protected if your situation changes. If you want to restart later, you’ll need another HIV test first, just as when you first started.

PrEP doesn’t prevent other STIs

The CDC is clear that PrEP reduces the chance of getting HIV but doesn’t prevent other sexually transmitted infections (STIs) or pregnancy. Chlamydia, gonorrhea and syphilis can still pass between partners. Other STIs also roughly double the risk of HIV (the calculator uses a factor of 2).

Condoms help protect against many other STIs, and you can ask for STI tests at your PrEP check-ups. Read more in HIV and STIs.

Regular check-ups

While you’re on PrEP, the CDC advises routine visits for HIV tests and prescription refills or shots. These visits catch problems early. In the rare case that a test comes back positive, see what to do after a positive HIV test.

What to do now

  • On PrEP and taking it as prescribed? Your risk from sex is very low. Keep going and keep your check-ups.
  • Missed doses and had a recent exposure? Contact a clinician now about PEP. The window is 72 hours.
  • Just started? Remember protection takes time to build. Use condoms in the meantime if you can.
  • Thinking of stopping? Plan it with your clinician first.
  • Not on PrEP yet? Read about PrEP and PEP and ask a sexual health clinic whether it’s right for you.