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You may have heard about medication you can take to prevent HIV. It’s called pre-exposure prophylaxis (PrEP). Here’s how it works, how to take it, and what to expect.

What Is PrEP?

PrEP is medicine you take to lower your risk of getting HIV from sex or injection drugs. When the medication builds up in your bloodstream, it stops HIV from taking hold in your body.

You may take PrEP if you have ongoing risk of getting infected and you don’t have HIV.

Is PrEP Safe and Effective?

If you take PrEP properly, it’s 99% effective against HIV infection from sex.

It’s less effective at preventing HIV infection if you inject drugs. Experts are still gathering information, but right now it looks like PrEP lowers your risk by about 74% if you inject drugs.

PrEP is most effective when you take it as prescribed. Your risk of being infected is higher if you don’t take it as directed.

It’s considered very safe. There are no reports of serious problems in people who take PrEP.

How Do I Take PrEP?

You can take PrEP as a daily pill or a bimonthly injection.

There are two pills approved for HIV prevention: emtricitabine plus tenofovir disoproxil fumarate (Truvada) and emtricitabine plus tenofovir alafenamide (Descovy). There’s one approved injection: cabotegravir extended-release injectable suspension (Apretude).

Your doctor will decide which option is best based on certain factors. For example:

  • If you have risk of HIV from sex and injection drugs, your doctor may recommend Truvada. 
  • If your risk is mostly through sex, they may recommend Descovy.
  • If you were assigned female at birth and your risk of infection is through vaginal sex, they may recommend an option other than Descovy.
  • If your risk is through sex and you weigh at least 77 pounds, your doctor may recommend the Apretude shot.
  • If you inject drugs, PrEP shots aren’t recommended.

PrEP isn’t for emergency use after HIV exposure. If you were exposed to HIV in the last 72 hours, talk to your doctor or emergency room or urgent care provider about post-exposure prophylaxis (PEP).

How Does PrEP Work?

PrEP works by blocking an enzyme HIV needs to make copies of itself, or replicate,  in your body.

After you start taking PrEP, it reaches a certain level in your bloodstream and in your mucus membranes that helps protect you from being infected.

How Fast Does It Work?

PrEP may start working after a few days. The medication needs time to reach a certain level in your body to get the most protection.

How long it takes to work depends on certain factors.

  • If you have receptive anal sex, also known as bottoming, it may take about 7 days of taking a pill every day.
  • If you have receptive vaginal sex or take injection drugs, it may take 21 days of daily pills.
  • It’s unclear how long it takes to work if you have insertive anal sex, also known as topping, or insertive vaginal sex.
  • Doctors aren’t sure how long it takes for PrEP shots to work.
  • If you take PrEP on demand, which means before planned sex, you need two pills 2 to 24 hours before you have sex, and then 1 pill 24 hours later, and another 24 hours after that.

Talk to your doctor about your sexual practices so they can tell you how long it will take for you to be protected.

How Do I Start?

You can get PrEP from any health care provider who is licensed to write prescriptions.

First, talk to your doctor to see what’s right for you. 

You’ll need an HIV test before starting PrEP. If you already have HIV, taking PrEP medication may create resistance to the drug, which may make it less effective for treating your HIV infection.

If you want to take a PrEP pill and you can’t do an in-office visit, you may be able to do it through telehealth. Your doctor may give you an appointment over the phone or through video. You can order an HIV testing kit, then mail in the specimen. Your doctor can help you decide if it’s right for you.

When you take PrEP, your doctor will see you regularly for office visits, HIV tests, prescription refills, or shots.

If you don’t have a doctor or if you want more information about HIV self-tests, visit the CDC’s HIV Service Locators.

What Are the Side Effects of PrEP?

Most people don’t have side effects from PrEP, but it’s possible you may get them.

Side effects include:

  • Diarrhea
  • Fatigue
  • Headache
  • Nausea
  • Skin reactions like rashes
  • Stomach pain

If you have side effects, they’ll probably get better with time. If they don’t improve, talk to your doctor.

What’s on-Demand PrEP and How Does It Work?

If you don’t have an ongoing risk of becoming infected, you may consider on-demand PrEP. This is a method where you only take pills when you’re planning to have sex in the next 24 hours.

Other names for on-demand PrEP include:

  • Event-driven PrEP
  • Intermittent PrEP
  • Non-daily PrEP
  • Off-label PrEP

With on-demand PrEP, you use a 2-1-1 schedule:

  • Take 2 pills 2-24 hours before sex.
  • Take 1 pill 24 hours after your first dose.
  • Take 1 pill 24 hours after your second dose.

If you’re a gay or bisexual man and you have anal sex without a condom, the 2-1-1 schedule may protect you from HIV. If you’re a heterosexual man or woman, if you’re transgender, or if you inject drugs, it’s not clear how well on-demand PrEP works.

The FDA hasn’t approved the 2-1-1 schedule. It only approved taking PrEP pills daily. The CDC doesn’t recommend the 2-1-1 schedule.

If you’re interested in on-demand PrEP, talk to your doctor. You can also find information at the CDC and health departments and organizations in the U.S., Europe, and Canada.

Show Sources

Photo Credit: Image Source/Getty Images


CDC: “How can I start PrEP?” “On-Demand Prep,” “PrEP Effectiveness,” “What is PrEP?”

Mayo Clinic: “PrEP: How effective is it at preventing HIV?”

New York State Department of Health AIDS Institute: “HIV Pre-exposure Prophylaxis (PrEP): Individualized HIV Prevention Frequently Asked Questions.”

Planned Parenthood: “What is PrEP?”

Washington Health Institute: “What is PrEP?”