PrEP: How It Works and Can You Still Get HIV?

PrEP greatly lowers the chance of getting HIV, but it has limits that are easy to miss. This page covers how PrEP works, how well it protects, and what to do if you are on it and worried.

What is PrEP and how does it work?

PrEP stands for pre-exposure prophylaxis. The CDC describes it as medicine, pills or shots, that reduces your chance of getting HIV. It is for adults and adolescents without HIV who may be exposed through sex or injection drug use.

It works by keeping enough medicine in your bloodstream to stop the virus from taking hold. In the CDC’s words, the right amount of medicine in the bloodstream can stop HIV from taking hold and spreading in your body. That is why it only works if you take it as prescribed.

Can you still get HIV while on PrEP?

Rarely, yes. The CDC says PrEP greatly reduces your chance of getting HIV, and estimates the risk is reduced by about 99% among men who have sex with men when it is taken daily or consistently. For heterosexual men and women, the CDC says there is no direct estimate for daily use. Its estimate for recent use, based on drug levels in the blood, is 88 to 90%. It also says that HIV acquisition is extremely rare when PrEP is taken that way, and that it was not observed in any of the studies it reviews. In clinical practice, though, a few new infections have been confirmed in HIV-negative people on PrEP with verified adherence.

Protection depends on taking it as prescribed. If you miss doses or stop, there may not be enough medicine in your bloodstream to block the virus.

Who is PrEP for?

The CDC says you may be a good candidate if you don’t have HIV and any of these apply:

• You’ve had anal or vaginal sex in the past 6 months and have a partner with HIV, haven’t consistently used a condom, or were diagnosed with an STI in the past 6 months
• You inject drugs and have an injection partner with HIV, or share needles or other equipment
• You’ve been prescribed PEP and report continued risk, or have used multiple courses of PEP

You can also choose PrEP if none of these apply. The CDC adds that PrEP can help prevent HIV while trying to conceive, during pregnancy and while breastfeeding.

What does PrEP not do?

PrEP doesn’t prevent other STIs or pregnancy. Condoms can help with both.

What does it involve?

You need an HIV test before starting, to confirm you don’t have HIV. After that you’ll have routine visits for follow-up, HIV tests and refills or shots. Any health care provider licensed to prescribe can prescribe PrEP, and in some states pharmacists can.

Dose, timing and type depend on the medicine and on you, so your prescriber decides these. The CDC says taking PrEP as prescribed is the only FDA-approved schedule, and that “on-demand” PrEP is not FDA-approved or recommended by the CDC.

What about cost?

The CDC says most insurance plans and state Medicaid programs cover PrEP, including the visits and lab tests needed to keep your prescription. Without insurance, some companies and some states run assistance programs.

I missed doses or stopped PrEP. What should I do?

Tell your prescriber, who can help you restart or choose another approach. The CDC says that if you’re thinking about stopping, talk to your provider first so you can stop safely and use other prevention methods.

If you’ve had a possible exposure within the last 72 hours, don’t wait for a routine visit. Go for care and ask about PEP. See HIV PEP: what to do in the first 72 hours. For testing and timing, our window periods guide explains when a test can detect HIV.

Do I still need to test if I’m on PrEP?

Yes. The CDC says people on PrEP receive regular HIV testing as part of their care. If you develop flu-like symptoms such as fever, rash or a sore throat, tell your prescriber, because testing can check whether it is HIV or something else.