How Effective Is PrEP for HIV and AIDS?

Medically Reviewed by Carmelita Swiner, MD on August 03, 2022

PrEP is medicine that helps protect you from the human immunodeficiency virus (HIV, which causes AIDS). It stands for pre-exposure prophylaxis.

  • “Pre” means before. You take the medicine before you think you might come into contact with the virus.
  • “Exposure” means contact with HIV.
  • “Prophylaxis” means treatment to stop infection in the first place. This is in contrast to treatment for people who either have signs of the virus in their blood (are HIV-positive) or already have symptoms linked to the illness.


PrEP stops HIV from taking hold in your body and spreading. If PrEP works as intended, you should not become HIV-positive.

For example, FTC/TDF (Truvada) is the main medication that doctors prescribe as a PrEP medication. It combines two medicines -- emtricitabine (FTC) and tenofovir disoproxil fumarate (TDF) -- to block an enzyme the virus needs to make copies of itself in your body.

FTC/TAF (Descovy) is the only other drug approved by the FDA for PrEP use. But doctors don’t yet know if it protects in the case of transmission through vaginal sex, so it may not work in those cases.

If you take it as your doctor directs, usually a pill a day, PrEP is quite effective against future HIV contact.

You should be able to protect yourself from HIV infection from sexual contact 99% of the time. If you inject drugs and share needles, PrEP isn’t as effective but can protect you in 74% of HIV exposures.

The medicine doesn’t work as well if you miss doses. You must take it as directed in order to get the protection.

You’re most likely to benefit from PrEP if you don’t have HIV (are HIV-negative), but your sexual activity or drug use raises your risk of getting it in the future.

Anyone can get HIV. You’re at greater risk for it if you:

  • Have more than one sexual partner
  • Have anal sex
  • Have vaginal or anal sex without using a condom
  • Have sex with anyone who has HIV
  • Share needles or syringes with someone who has HIV
  • Have (or have had recently) another sexually transmitted infection (STI), such as chlamydia, syphilis, gonorrhea, or herpes

So if you’re HIV-negative, you may want to talk to your doctor about PrEP if you have any of the “risk factors” listed above.

PrEP can help protect both you and your baby if you plan to get pregnant from a partner with HIV. It helps block the virus during pregnancy and while breastfeeding. Talk to your doctor about the best way to do this.

No. A vaccine causes your body to make special substances called “antibodies” that will fight against a disease-causing germ, like a virus, long after you take the dose. PrEP protects you against HIV only as long as you continue to take it. The effect disappears once you stop taking the medicine.

PrEP also doesn’t prevent pregnancy or stop you from getting other sexually transmitted diseases, or STDs. The best way to protect against STDs like gonorrhea and chlamydia is to wear a condom.

Side effects are few and usually go away. Nausea is most common and occurs in about 9% of users. You might also have a headache. In most cases, these effects disappear after a few weeks of use.

There are no serious or life-threatening side effects.

Show Sources

SOURCES: “What Is Pre-exposure Prophylaxis (PrEP)?”

CDC: “Immunization: The Basics,” “PrEP.” “Pre-Exposure Prophylaxis.” “The Basics.”

University of California, San Francisco: “Descovy (FTC/TAF) for PrEP.”

U.S. National Library of Medicine: “Pre-Exposure Prophylaxis (PrEP).”

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