How Pre-Exposure Prophylaxis (PrEP) Works to Prevent HIV

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smiling middle aged Caucasian male holding PrEP HIV pill

You’ve probably gotten a flu shot to avoid getting sick in the winter. Fortunately, we can now do the same for HIV. Pre-exposure prophylaxis, or PrEP, uses anti-HIV medications to prevent HIV infection.

When someone is HIV-negative but at high risk of contracting it, taking PrEP daily can substantially lower this risk.

Understanding the PrEP Pill

At this time, Truvada is the only PrEP medication approved by the U.S. Food and Drug Administration (FDA). Truvada is a combination medication made up of two drugs–tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC). It comes in a pill form and only needs to be taken once a day. Truvada must be started before coming into contact with the virus.

HIV uses an enzyme to allow the virus to replicate and spread to other cells throughout your body. PrEP works by blocking that enzyme, making it difficult for the virus to copy itself and lead to an infection. PrEP can take 7 to 20 days to reach full effectiveness and can reduce the risk of getting HIV from sex by over 90% . But the level of protection wanes when PrEP is not taken every day as directed.

You may experience some mild side effects, like nausea, headaches, or dizziness, when starting PrEP, but most people report the side effects improve over time. In very rare cases, more serious problems may develop, including issues with liver and kidney function and bone density. Let your doctor know if you have existing liver or kidney disease or osteoporosis, as well as any other medications you are currently taking.

Determining if You’re a Candidate for PrEP

You must be HIV-negative before starting PrEP. Your doctor will do a blood test to check. However, you should be mindful there’s a window of time lasting several weeks when you could be newly infected with HIV but not receive a positive result on an HIV test. Talk to your doctor about any recent potential risk of HIV exposure and how to recognize the early signs of an HIV infection to determine if you should have additional testing before starting PrEP.

You’ll need to have an honest conversation with your doctor to see if you fall into a high-risk category for contracting HIV. According to the Centers for Disease Control and Prevention (CDC), PrEP should be considered for HIV prevention for the following groups:

  • Anyone who has regular sex with an HIV-positive partner

  • Gay or bisexual men and transgender men and women who are not in a mutually monogamous relationship and have had sex without a condom, or who have had a sexually transmitted disease (STD) in the past six months

  • Heterosexual men and women who are not in a mutually monogamous relationship and have had sex with partners of unknown HIV status without using a condom

  • Men or women who inject drugs and share needles or who have been in treatment for drugs in the last six months

Using PrEP as Part of an HIV Safety Plan

Preventing HIV with PrEP can be very effective, but it’s not 100%. So when it comes to HIV safety, there are additional things you can do to lower your risk:

  • Use condoms regularly. Not only will this give you added HIV protection, but it can guard against all the other STDs that PrEp doesn’t cover.

  • Get tested and see your doctor regularly. If you’re using PrEP, you’ll need to visit your doctor at least every three months for HIV testing. This is a good time to let your doctor know how well you are adhering to your medication schedule and discuss any questions or concerns you may have.

  • Make safer decisions when it comes to your sex life. Ask your partner to get tested for HIV and other STDs. Limit your number of partners.

  • Don’t inject illegal drugs or share needles.

Though we’ve come a long way with treating HIV, there still isn’t a cure. That’s why HIV prevention with PrEP has been an exciting advance in this area of medicine. Research remains underway to see if other anti-HIV medications can be used as PrEP, as well as if longer lasting methods of delivering the medication, such as with injections and implants, are viable treatment options. For more information about starting PrEP, talk to your doctor.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2019 Jul 11
  1. Preexposure
    Prophylaxis for the Prevention of HIV Infection in the United States-2017
    Update Clinical Practice Guideline. Centers for Disease Control and Prevention.
  2. Pre-Exposure
  3. PrEP. Centers for
    Disease Control and Prevention.
  4. The Basics of HIV
    Prevention. AIDSInfo.
  5. What is PrEP? Planned
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