Pre-exposure Prophylaxis (PrEP)

Pre-exposure prophylaxis (PrEP) is an HIV prevention method where HIV-negative individuals at risk of HIV infection take a daily medication before being exposed to the virus. “Pre-exposure” means before coming into contact and “prophylaxis” stands for preventative treatment.1 At the time this article was updated, only two medications, under the brand names Truvada® and Descovy®, have been approved by the U.S. Food and Drug Administration (FDA) for daily use to prevent HIV infection.2 In this article, “PrEP” refers to both Truvada® and Descovy® unless specified.

How Does PrEP Work?

PrEP, both Truvada® (emtricitabine and tenofovir disoproxil fumarate) and Descovy® (emtricitabine and tenofovir alafenamide), are combinations of two different anti-viral drugs that lower the chance of HIV-1 infection by interfering and stopping the virus from duplicating and spreading in the host’s body.1 PrEP is not a vaccine and must be taken once each day consistently. A certain amount of PrEP must be present in the bloodstream for it to be effective. PrEP reaches maximum protection from HIV for receptive anal sex at about 7 days of daily use. Truvada® reaches maximum protection at about 21 days of daily use for receptive vaginal sex and injection drug use.2 Descovy® has not been studied for HIV prevention in the case of receptive vaginal sex.

How Effective Is PrEP?

PrEP is a highly effective HIV prevention method when taken daily and consistently. According to current studies, PrEP reduces the risk of HIV infection through sex by approximately 99% when taken consistently.2 For people who inject drugs, PrEP reduces the risk of HIV infection by at least 74% when taken consistently.2 Inconsistent use, or missing a dose, decreases the effectiveness of PrEP. The risk of getting HIV from sex can be further lowered when PrEP is combined with other preventive methods, such as condom use.2 PrEP is one of the most effective HIV prevention methods when used correctly. It is the individual’s responsibility to follow their health professional’s instructions and take PrEP as indicated. 

Who Is PrEP Suitable for?

PrEP is suitable for people who do not have HIV, who weigh at least 35 kg (77 lbs), and who are at high risk of HIV exposure through sex or drug injections.1 Truvada® for PrEP is recommended for all people at risk of HIV through sex or injecting drugs. Descovy® for PrEP is recommended for people at risk of HIV only through sex, excluding people who have receptive vaginal sex.

According to the Centers for Disease Control and Prevention’s (CDC) guidelines, people at high risk of HIV infection include, but are not limited to, the following:

Individuals who have had anal or vaginal sex within the past 6 months and:

  • Have an HIV-positive sexual partner (especially if the partner has an unknown or detectable viral load).
  • Have not consistently used a condom.
  • Have contracted an STI within the past 6 months.

Individuals who inject drugs and:

  • Have an HIV-positive injection partner.
  • Share needles, syringes, or other equipment to inject themselves.

Individuals who have been prescribed non-occupational post-exposure prophylaxis (PEP) and:

  • Report continued risk behavior.
  • Have used multiple courses of PEP.

Individuals need to be tested for HIV before taking PrEP and at least every three months while taking PrEP. Individuals need to maintain HIV-negative status before and while taking PrEP since PrEP by itself is not a complete treatment for HIV.

Do not take Truvada® or Descovy® for PrEP if you are HIV-positive or if you do not know your HIV status.Do not take Descovy® for PrEP if you are a person with a vagina or are at risk of HIV from receptive vaginal sex.4 Do not take PrEP if you think you have been exposed to HIV. Post-exposure prophylaxis (PEP) is an option for someone who may have recently been exposed to HIV. PrEP does not prevent other sexually transmitted infections (STIs). Consistent condom use is recommended when taking PrEP to further lower the risk of HIV and other STIs.

Is PrEP Safe?

There have been no significant health effects observed in people who are HIV-negative and have taken PrEP for a prolonged period of time. If the individuals have hepatitis B virus (HBV) infections, the HBV infection may worsen after they stop taking PrEP. 2 Health care providers will test individuals for HBV before and while taking PrEP.

Common side effectsNausea
Stomach pain
Decreased weight
Stomach pain
Possible side effectsKidney problems
Lactic acid buildup in the blood
Liver problems
Bone problems
Kidney problems
Lactic acid buildup in the blood
Liver problems 

How Do I Start PrEP?

Talk to your health care provider to see if PrEP is right for you. PrEP can only be prescribed by health care providers to HIV-negative individuals who are suitable for PrEP. Individuals should follow up with their doctors at least once every three months to test for HIV, HBV, liver/kidney functions, and other tests as necessary.1 Take PrEP as instructed, or daily, with or without food. If you miss a dose, take the next dose as soon as possible. Do not take two doses at once to compensate. Do not stop taking PrEP without consulting with a health care professional first. The cost of PrEP varies.

Concluding Remarks

Pre-exposure prophylaxis (PrEP) is a safe and highly effective HIV prevention method. In order to ensure maximum protection, PrEP must be taken daily on a regular basis or as instructed. Coupling PrEP with another prevention method, such as a condom, could reduce the chances of HIV infection even further. PrEP should only be taken by individuals who do not have and have not been exposed to HIV. Depending on the person’s individual situation, a health care provider will recommend whether Truvada® or Descovy® can be used for PrEP. Consult with a health care professional before taking PrEP. Take PrEP as instructed and make sure to follow up with your doctor regularly. If you are experiencing side effects, or you want to stop taking PrEP, talk to your health care provider.


  1. “Information on HIV/AIDS Treatment, Prevention, and Research.” AIDSinfoU.S. Department of Health and Human Services, 2020.
  2. “PrEP | HIV Basics.” Centers for Disease Control and Prevention, U.S. Department of Health and Human Services, 2019.
  3. TRUVADA for PrEP. Web. Gilead Sciences, 2020.
  4. DESCOVY for PrEP. Web. Gilead Sciences, 2020.

Last Updated: 4 March 2020.