According to the CDC:
“PrEP” stands for Pre-Exposure Prophylaxis. The word “prophylaxis” means to prevent or control the spread of an infection or disease. The goal of PrEP is to prevent HIV infection from taking hold if you are exposed to the virus. This is done by taking one pill every day. These are some of the same medicines used to keep the virus under control in people who are already living with HIV.
PrEP medicine is not injected into the body and does not work the same way as a vaccine. A vaccine teaches your body to fight off infection for several years. For PrEP, you take a pill every day by mouth. The pill that was shown to be safe and to help block HIV infection is called “Truvada” (pronounced tru vá duh). Truvada is a combination of two drugs (tenofovir and emtricitabine). If you take PrEP daily, the presence of the medicine in your bloodstream can often stop HIV from taking hold and spreading in your body. If you do not take PrEP every day, there may not be enough medicine in your bloodstream to block the virus.
CDC recommends that PrEP be considered for people who are HIV-negative and at substantial risk for HIV.
For sexual transmission, this includes anyone who is in an ongoing relationship with an HIV-positive partner. It also includes anyone who 1) is not in a mutually monogamous* relationship with a partner who recently tested HIV-negative, and 2) is a gay or bisexual man who has had anal sex without a condom or been diagnosed with an STD in the past 6 months; or heterosexual man or woman who does not regularly use condoms during sex with partners of unknown HIV status who are at substantial risk of HIV infection (e.g., people who inject drugs or have bisexual male partners).
For people who inject drugs, this includes those who have injected illicit drugs in the past 6 months and who have shared injection equipment or been in drug treatment for injection drug use in the past 6 months.
For heterosexual couples where one partner has HIV and the other does not, PrEP is one of several options to protect the uninfected partner during conception and pregnancy.
People who use PrEP must be able to take the drug every day and to return to their health care provider every 3 months for a repeat HIV test, prescription refills, and follow-up.
* Mutually monogamous means that you and your partner only have sex with each other and do not have sex outside the relationship.
In several studies of PrEP, the risk of getting HIV infection was much lower—up to 92% lower—for those who took the medicines consistently than for those who didn’t take the medicines.
Some people in clinical studies of PrEP had early side effects such as an upset stomach or loss of appetite, but these were mild and usually went away in the first month. Some people also had a mild headache. No serious side effects were observed. You should tell your health care provider if these or other symptoms become severe or do not go away.
Visit the CDC's web page for more information about PrEP.