PrEP (pre-exposure prophylaxis) is medicine that HIV-negative people take before possible exposure to keep HIV from taking hold. It comes as a daily pill or a long-acting shot. Taken as prescribed, it cuts the risk of getting HIV from sex by about 99% CDC. It prevents HIV. It doesn't treat it and doesn't cover other STIs.
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 — "prophylaxis" means something you take to prevent disease before it happens. You keep a protective level of anti-HIV medicine in your body so that if the virus does get in, it can't establish a lasting infection.
HIV has to get inside your cells and copy itself to spread through your body. The drugs in PrEP block the enzyme the virus needs to make new copies of itself. If you already have that medicine circulating in your blood and the tissues HIV tries to enter, the virus gets stopped before it can set up shop. Timing matters because the protection comes from having enough drug on board. Popping a pill after the fact won't do it.
This is the key difference between PrEP and PEP. PrEP is taken before exposure, on an ongoing basis, by people who expect to have possible HIV exposures over time. PEP (post-exposure prophylaxis) is the emergency option you start after a single possible exposure — like a condom break or a needle stick — and it has to begin quickly. The two are not interchangeable. If you've already had an exposure and you're not on PrEP, PEP is the route, and you can read more about how that works at earlier hiv treatment can help prevention.
How well PrEP works (the numbers)
When 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 depend heavily on consistency. PrEP only protects when there's enough drug in your system, so missed doses chip away at how well it works.
Protection doesn't switch on the moment you take your first pill. The body needs time to build up protective drug levels in different tissues, and that timeline varies by how you might be exposed:
- About 7 days of daily dosing for protection during receptive anal sex.
- About 21 days of daily dosing for protection during receptive vaginal sex and for injection drug use.
Until you reach that window, use other protection like condoms. This is also why PrEP fails people who treat it as morning-after coverage, since it isn't built for that.
How to use PrEP and who it's for
PrEP is for people who are HIV-negative and have an ongoing chance of exposure — through sex with a partner whose status is positive or unknown, multiple partners, a recent STI, or shared injection equipment. You don't need a "perfect" reason. If HIV is a realistic concern in your life, you're a candidate. There are a few formulations, and which one fits depends on how you're at risk and your preferences.
| 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); a choice for those who'd rather not take a daily pill |
The daily pills work by keeping a steady drug level when taken every day. The injectable, Apretude, is given on a schedule by a clinician and is an option for people who struggle with a daily routine. You can learn more about the original pill at truvada - a pill that may prevent hiv infection.
In practice, starting PrEP means an HIV test first. You have to be HIV-negative to begin, because taking PrEP alone with undiagnosed HIV can cause problems. After that, you'll have regular check-ins while you're on it to confirm you're still negative and screen for STIs. The visits are routine and quick once you're settled into the rhythm.
Cost and how to get PrEP
You can get PrEP from a primary care doctor, a sexual health clinic, or a telehealth service — many people start it without ever sitting in a waiting room. Programs exist to cover the cost for people who are uninsured or underinsured, so price shouldn't be the thing that stops you. Ask the prescriber or clinic directly about assistance; staff at PrEP-focused clinics deal with this every day and can point you to the right program.
For a full how-to on treatment regimens, dosing, and what the assistance programs look like, work through your prescribing clinic or telehealth provider. That's a separate, detailed conversation from this PrEP 101 overview.
What PrEP does NOT protect against
PrEP is HIV-specific. It does nothing against other sexually transmitted infections — chlamydia, gonorrhea, and syphilis can all still be passed even when PrEP is working perfectly against HIV. It also offers no protection against pregnancy. PrEP is one layer of protection, not the whole wall.
A common mistake is assuming that being on PrEP means you no longer need to test for anything. People on PrEP should keep testing regularly for STIs, both because PrEP doesn't cover them and because regular screening is built into staying on PrEP safely.
How PrEP fits with other prevention
Think of prevention as a toolkit rather than a single device. PrEP handles HIV; condoms reduce the risk of HIV plus the STIs PrEP can't touch, and they prevent pregnancy; vaccines protect against infections like hepatitis B and HPV. Routine STI testing ties it all together so anything that slips through gets caught and treated early.
If you're on PrEP or thinking about it, make screening part of the plan and get tested on a regular schedule. And if you've had a specific exposure, knowing the right timing matters — read up on when to test after exposure so you don't test too early and get a falsely reassuring result.
When to talk to a clinician
Reach out if HIV is a realistic possibility in your life and you'd feel safer with a layer of protection in place. There's no threshold of risk you have to "prove" first. Bring it up if you have a partner living with HIV, if you've recently had an STI, if you share injection equipment, or if you simply want the peace of mind. A clinician will confirm you're HIV-negative, talk through which formulation fits, and set up the check-ins.
If you think you've already been exposed to HIV and you're not on PrEP, that's a PEP situation, and it needs to start quickly — don't wait for a PrEP appointment CDC. Call a clinic, urgent care, or emergency department the same day.