PrEP doesn't protect you the moment you swallow the first pill. Daily oral PrEP reaches its maximum protection in about 7 days for receptive anal sex, and about 21 days for receptive vaginal sex and injection drug use CDC PrEP. Until then, keep using condoms. The drug needs time to build up in each kind of tissue.
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 — medicine that HIV-negative people take before a possible exposure to keep HIV from taking hold. The active drugs travel through your bloodstream and into the genital, rectal, and other tissues where HIV would first try to set up an infection. When enough drug is sitting in those cells, the virus can't make copies of itself, so an exposure that might otherwise have led to infection goes nowhere.
The drug doesn't reach every tissue at the same speed. Rectal tissue takes up the medicine quickly, which is why protection for receptive anal sex arrives in roughly a week. Vaginal and cervical tissue, plus the blood compartment relevant to injection drug use, take about three weeks to reach protective levels. This isn't about how careful you are. It's how the body distributes these specific drugs.
PrEP is different from PEP, the emergency option you take after a single possible exposure. PEP is a sprint started within hours; PrEP is an ongoing plan. If you've already had a risky exposure and aren't on PrEP, have the PEP conversation fast 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, it lowers the risk by at least 74% when taken as prescribed. Those numbers assume consistent use. Miss doses and the drug level in your tissues drops, and your protection with it.
The timeline matters here. Starting PrEP today doesn't backdate your protection, and a single tablet won't cover an exposure tomorrow morning. The protection you read about is what you get once the medicine has had its ramp-up period and you've stayed on schedule.
How to use PrEP and who it's for
There are two daily pills and one shot, and which one fits depends on how you're at risk.
| Option | Form | 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 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); an option if you'd rather not take a daily pill |
The injectable, Apretude, is a good fit for people who know they'll struggle to take a pill every day. The daily pills are well-studied across a wide range of people. If you're not sure you qualify or which form makes sense, the who should take prep? eligibility guide walks through who benefits most.
In practice, starting PrEP begins with an HIV test — you have to be HIV-negative to start — followed by regular check-ins while you're on it. Those visits confirm you're still HIV-negative, check kidney function, and screen for other STIs. A common mistake is treating PrEP like a morning-after fix. It only works on an ongoing schedule, and it doesn't cover other infections, so people on PrEP still need routine testing.
What PrEP costs and how to get it
You can get PrEP from a primary care clinic, a sexual-health clinic, or a telehealth service — many people now start it entirely online with labs done locally. Cost stops a lot of people, but most insurance covers PrEP, and patient-assistance and copay programs exist to cover the drug and even the lab visits for people who'd otherwise pay out of pocket. If money is the barrier, say so at your visit; the clinic almost always knows a program that helps.
What PrEP does NOT protect against
PrEP guards against HIV, and only HIV. It does nothing against chlamydia, gonorrhea, or syphilis, and it doesn't prevent pregnancy. Ongoing testing is part of being on PrEP. People sometimes drop condoms once they start PrEP and are surprised by a gonorrhea or syphilis diagnosis a few months later.
- PrEP does not prevent chlamydia, gonorrhea, or syphilis. These bacterial STIs spread through the same contact PrEP can't touch.
- PrEP does not prevent pregnancy; you'll need separate contraception if that's a concern.
- PrEP does not work as a one-off after exposure — that's PEP's job.
How PrEP fits with the rest of your prevention
Think of PrEP as one strong layer rather than the whole roof. Condoms add protection against the STIs PrEP misses. Routine STI screening catches infections early, before they cause complications or spread — if you're on PrEP, build a regular schedule to get tested. If you've had a specific risky encounter and are wondering how soon a test will pick something up, the guide on when to test after exposure explains the timing for each infection.
Prevention also works at the community level. When people living with HIV are diagnosed and treated effectively, the virus becomes untransmittable through sex, so earlier hiv treatment can help prevention is part of the same picture as your own PrEP. Vaccines round it out: hepatitis B and HPV vaccination protect against infections PrEP and condoms don't fully cover.
When to talk to a clinician
Reach out before you start, not after a scare, if you think PrEP might fit your life: a new partner whose status you don't know, a partner living with HIV, inconsistent condom use, or injection drug use. Talk to a clinician too if you've started PrEP but keep missing doses, if you've had a possible exposure during the ramp-up window before protection kicked in, or if you develop symptoms of another STI. And if a high-risk exposure just happened and you're not yet protected, ask about PEP the same day — hours matter there.