What Is HIV Pre-Exposure Prophylaxis (PrEP)?

Preventive Strategy Used to Reduce the Risk of Getting HIV

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HIV pre-exposure prophylaxis (PrEP) is a preventive strategy in which the daily use of the antiretroviral drug Truvada (emtricitabine + tenofovir disoproxil fumarate) or Descovy (emtricitabine + tenofovir alafenamide) can reduce the risk of getting HIV by up to 99%.

Pile of Truvada pills
 MarcBruxelle / Getty Images

PrEP is not intended for everyone but is rather used in people who are at high risk of HIV infection. This includes serodiscordant couples in which one partner is HIV-positive and the other is HIV-negative.

Taking PrEP does not mean you can stop using condoms or avoid traditional risk-reduction strategies. The drugs used for PrEP do not prevent other sexually-transmitted or blood-borne infections, including hepatitis C.

Truvada was the first drug approved for PrEP in 2012. Descovy (which includes an "improved" version of tenofovir that is less impactful on the kidneys) was approved in October 2019.

Uses

PrEP is solely used in people who are at high risk of getting HIV. This is because PrEP requires a commitment to daily pill-taking as well as routine blood tests and evaluations for the strategy to work.

According to guidance from the U.S. Preventive Service Task Force (USPSTF), PrEP is recommended for the following high-risk groups:

Risk Group Risk Category
Men who have sex with men (MSM) Who are in a serodiscordant relationship
  Who do not use condoms consistently for receptive or insertive anal sex
  Who have had a sexually transmitted disease (STD) within the past six months
Heterosexual women and men Who are in a serodiscordant relationship
  Who do not use condoms consistently with partners of unknown status or who at high risk of having HIV (such as an injecting drug user)
  Who have had an STD within the past six months
Injecting drug users Who have shared needles or other injecting drug paraphernalia (including spoons, cotton balls, or water)
  Who are in risk of getting HIV from sex

In addition, adolescents or adults considering PrEP must weigh at least 77 pounds (35 kilograms) to be candidates for treatment.

Because the dose of Truvada and Descovy is fixed and cannot be modified, people weighing less than 77 pounds would need to use other traditional preventive strategies.

Although people at low risk of infection can benefit from PrEP, there are risks associated with treatment (see "Side Effects" below) as well as potential costs. For these individuals, condoms and other traditional preventive strategies may be a more practical solution.

Before Taking

Prior to starting PrEP, your doctor will need to confirm that you do not have HIV. This requires a simple blood test, either in the form of a rapid HIV test (which can return results in around 20 minutes) or a standard point-of-care test (which usually returns results in a couple of days).

In addition, the doctor will order blood tests to check your kidney function. Because Truvada and Descovy can impair kidney function in some people, the test provides a baseline reading by which to monitor for any adverse effects.

Truvada and Descovy should not be used in anyone with a creatinine clearance of less than 30 milliliters per minute (mL/min). Values under 30 mL/min are an indication of severe kidney impairment.

You may also be screened for sexually transmitted infections, including hepatitis B. A pregnancy test is also recommended if you are in your child-bearing years.

Precautions and Considerations

PrEP should be delayed in anyone who has signs of a recent HIV infection. This is because HIV tests are designed to detect antibodies, which can take up to a month to develop after exposure to the virus. During this so-called "window period," an HIV test can return a false-negative result, meaning that you have HIV even if the test says otherwise.

Symptoms of acute HIV infection (also known as acute retroviral syndrome) are often flu-like and include:

You should also delay PrEP if you've had a possible exposure to HIV within the past month, either through unprotected sex, sex with a partner of unknown status, or shared needles.

Dosage

When used for PrEP, the recommended dose of Truvada and Descovy is one tablet daily taken with or without meals. The tablet must be taken daily to achieve optimal protection. 

It can take up to three weeks for the drug to reach the maximum protective concentration. According to the Centers for Disease Control and Prevention (CDC), this can vary by the mode of transmission:

  • Anal sex: 7 days of uninterrupted use
  • Vaginal sex: 21 days of uninterrupted use
  • Injecting drug use: 21 days of uninterrupted use

According to the CDC, PrEP reduces the risk of HIV from sexual contact by 99% if taken daily. Among injecting drug users, PrEP reduces the risk by at least 74%.

Modifications

The CDC allows for the tenofovir on its own, in the form of the antiretroviral tablet Viread (tenofovir disoproxil fumarate), as an alternative for heterosexual men and women and people who inject drugs. The same is not true for MSM who remain at the highest risk of HIV in the United States.

In 2018, the International Antiviral Society USA further endorsed the use of PrEP "on-demand" for MSM only. This is a strategy not endorsed by the CDC but one that has gained the approval of many public health authorities concerned about the continued high infection rate in gay and bisexual men.

PrEP on-demand is supported by research published in the New England Journal of Medicine, which showed that in high-risk MSM (defined as having unprotected anal sex with at least two partners during the past 2 months) were 84% less likely to get HIV if they took PrEP before, during, and after sex.

How to Take PrEP On-Demand

The International Antiviral Society USA recommends PrEP on-demand on a "2-1-1" schedule, namely:

  1. Two tablets of Truvada are taken 2 to 24 hours before sex.
  2. One tablet is taken 24 hours after the first dose.
  3. A final tablet is taken 24 hours later.

If the partners engage in sex before the end of the 48-hour period, they should continue to take PrEP every 24 hours until 48 hours after their last sexual episode.

Follow-Up Testing

If you decide to pursue PrEP as a preventive strategy, you will be given a three-month prescription of Truvada or Descovy. Every 3 to 6 months thereafter you will need to undergo follow-up testing and evaluations per CDC guidance:

  • Every 3 months: Repeat HIV testing, pregnancy testing, and STD testing for people with symptoms of an STD
  • Every 6 months: Repeat creatinine clearance testing
  • Every 12 months: Evaluate whether PrEP should continue based on self-reported adherence, individual risk assessment, treatment tolerance, and other factors

How to Take and Store

Truvada and Descovy can be taken with or without food but should not be split, crushed, or chewed as this can affect the absorption of the drug.

Truvada and Descovy can be store safely at room temperature, ideally around 77°F (25°C). Keep the pills in their original light-resistant container. Do not remove the moisture-retardant sachet, and never store the pills on a sunny windowsill or in your glove compartment. Discard any expired tablets.

If you miss a dose, take it as soon as you remember. If it is near the time of your next dose, skip the original dose and continue as normal. Do not double up doses.

Side Effects

As with all drugs, Truvada and Descovy can cause side effects. For the most part, they will be mild and transient and usually resolve within a week or so as your body adapts to treatment. In rare cases, the drugs can cause severe side effects, often in people with pre-existing health conditions.

Common

Truvada and Descovy have slightly different mechanisms of action but many of the same side effects. Because Descovy delivers tenofovir at a significantly lower dose, the incidence of side effects are lower (between 2% and 5%) compared to Truvada (between 5% and 9%).

The most common side effects by order of frequency include:

Truvada
  • Fatigue

  • Nausea

  • Diarrhea

  • Dizziness

  • Rash (usually mild)

  • Headache

  • Insomnia

Descovy
  • Diarrhea

  • Nausea

  • Headache

  • Fatigue

  • Stomach pain

Severe

Severe side effects are rare but can occur. The most concerning is the risk of kidney impairment. This typically affects people with pre-existing kidney disease but can also occur in those who take medications actively excreted by the kidneys (see "Drug Interactions" below).

On rare occasions, this can lead to acute kidney failure, the symptoms of which include:

In most cases, kidney function will return to baseline levels once treatment is stopped.

Warnings and Interactions

Both Truvada and Descovy carry a black box warning advising consumers that a severe flare-up of liver symptoms can occur in people with hepatitis B following the discontinuation of treatment. Black box warnings are the highest level of advisement from the U.S. Food and Drug Administration (FDA).

Although not contraindicated for use, Truvada and Descovy should be used with extreme caution in anyone with hepatitis B. If used, liver function tests should be routinely repeated. Hepatitis B treatment should also be considered.

Truvada and Descovy are considered safe for use during pregnancy. Both animal studies and human surveillance data have shown no increase in the risk of fetal harm compared to the general population. Breastfeeding is also presumed to be safe.

A 2016 study in PLoS Medicine reported that mothers on PrEP only transmit miniscule amounts of tenofovir and emtricitabine through breastmilk and that nine out of 10 have no detectable traces at all.

With that said, due there is a lack of long-term safety research, it is important to speak with your doctor about the benefits and risks of PrEP if you are pregnant, planning to get pregnant, or breastfeeding.

Drug Interactions

There are few major drug interactions associated with either Truvada or Descovy. The biggest concern is the risk of kidney-related side effects when taken with drugs that also rely on the kidneys for excretion.

These include the following classes of drugs:

Advise your doctor if you take any of the following A dose adjustment or change of treatment may be needed if there is evidence of kidney impairment.

To avoid interactions, always advise your doctor about any drugs you take, whether they are prescription, over-the-counter, herbal, nutritional, or recreational.

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