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How Effective is PrEP?

How Effective is PrEP?

Updated on:
December 21, 2021

Are you sexually active and want to minimize your exposure to HIV? Today, we have more tools than ever to prevent HIV and reduce harms associated with having an active sex life.

What is PrEP? Pre-exposure prophylaxis (PrEP) medication works to prevent HIV infection in those who are HIV negative and at higher risk for HIV. PrEP combines two drugs in one pill that is taken every day.

When PrEP is in your bloodstream, it can stop HIV from taking hold and spreading, keeping you HIV negative.

Here are the main points if you don’t feel like skimming:

  • When PrEP is taken as daily prescribed, it reduces the risk of getting HIV by 99%.
  • Insertive partners (those topping) have a lower chance of getting HIV, while those receiving (bottoming) have a higher chance.
  • For daily PrEP users, studies have shown that it can reduce HIV risk by 84% from drug injection.
  • When taken correctly, PrEP on-demand has shown to have an overall efficiency of 86% in preventing HIV among MSM (men who have sex with men).
  • For anal sex, PrEP is highly effective after taking it for 7 days consistently.
  • For vaginal/frontal sex, PrEP takes about 21 days to reach desired concentrations for protection.
  • If PrEP is taken four times a week, it's 99% effective at preventing HIV infection through anal sex. If taken only two times a week, efficacy drops to 75%.

This method of protection currently comes in three types of approved pills (Truvada, generic Truvada, & Descovy medication). All of which offer a similar level of protection when taken daily (99%).

PrEP effectiveness

PrEP is highly effective in preventing HIV. When PrEP is taken as daily prescribed, it reduces the risk of getting HIV by 99%. PrEP is even more effective when combined with other protections, such as condoms. Though it can also be prescribed as PrEP on demand.

You should consider taking PrEP if:

  • You have sexual intercourse with multiple partners
  • You inject drugs and/or use drugs during sex
  • You had condomless sex without knowing your partner's HIV status

When taken as prescribed, PrEP is safe and potent. However, PrEP may cause minor side effects in a small number of patients in their first days such as:

  • nausea
  • diarrhea
  • fatigue
  • headache
  • and stomach pain

PrEP for bottoming

PrEP is one of the best options for receptive anal sex. When taken daily for seven days, PrEP is extremely effective in preventing HIV. As explained above, it's okay to miss a few doses, however, if you find that sticking to daily PrEP is challenging, speak to your healthcare provider for some strategies that can help. For example, prescribing PEP could make more sense in emergency situations.

Be aware that each dose you miss reduces the efficacy of PrEP. It's best to follow a daily dosage schedule if you want maximum protection.

PrEP for topping

Insertive partners have a statistically lower chance of getting HIV. However, this doesn't mean that you're risk-free. In deciding whether to take PrEP as a top, there are some things to consider.

PrEP for drug injection

Canadian PrEP guidelines recommend that PrEP be considered by those who inject drugs if they are at a high risk for HIV. People who share injection drug equipment, like needles, cookers, filters, and water, are at higher risk of HIV transmission. For those that take PrEP on a consistent daily basis due to risk from drug injection, studies have shown that it can reduce HIV risk by 84%

Though the best way to reduce risk during drug injection is to use new equipment every time. Do not re-use or share your injection equipment.

If you inject drugs and are thinking about PrEP, it’s important to consider your ability to safely store the medication, and to make plans to take it daily.

PrEP on-demand

When taken correctly, PrEP on-demand has shown to have an overall efficiency of 86% in preventing HIV among MSM. 

Note that PrEP on-demand has only been approved for people having anal sex - those having vaginal/frontal sex should not consider PrEP on-demand due to differences in the time it takes PrEP to act in body tissues

If you take PrEP on-demand, it's crucial to stick to the prescribed dosing guidelines. This is due to PrEP losing protective efficiency when it’s taken less often. The most efficient PrEP on-demand routine is the following:

  • Take two pills 2 to 24 hours before sexual contact, and
  • Take one pill/day, for two days. Starting 24 hours after your step 1 dose (total of 2 pills, one pill/day)

How long does PrEP take to be effective?

PrEP is not immediately beneficial. Like any other medication, it takes time for it to take effect.

  • For anal sex: PrEP is highly effective after taking it for 7 days without missing a dose. 
  • For vaginal / frontal sex: it takes PrEP about 21 days to reach desired concentrations.

What causes PrEP to not be effective?

Research has found that PrEP is less effective when it isn't taken every day. This lack of protection is because there isn't enough of the medicine in your body to prevent HIV from spreading.

If PrEP is taken four times a week, it's 99% effective at preventing HIV infection through anal sex. If taken only two times a week, efficacy drops to 75%.

As PrEP's efficiency decreases when doses are missed, adherence (taking your medication as prescribed) is essential to maintain maximum levels of protection.

Is your partner HIV negative? Are they correctly taking PrEP? Do you want total control over preventing HIV, or are you willing to accept the small risk of getting HIV as a top? Are you using a condom correctly each time you have sexual intercourse?

These are all questions to think about before deciding whether to take PrEP.

Resources:

https://www.catie.ca/en/fact-sheets/prevention/pre-exposure-prophylaxis-prep

Reviewed by:
Thomas Trombetta

Thomas is passionate about gender and sexuality liberation social movements. Before beginning his work with Freddie, Thomas studied Sociology and Global & Development Studies at the University of Alberta, after which he began working with marginalized communities. In previous roles, Thomas was involved in queer and trans health education, PrEP health promotion, community-based research, HIV education, and LGBTQ2S+ advocacy.