Event-based prep: How to take it
PrEP (pre-exposure prophylaxis) is effective as a means of preventing HIV infection, but the dosage regimens can vary. Most people are aware of the daily dosing regimen, in which one dose of PrEP is taken daily, but there is another way of taking PrEP also known as event based PrEP or on demand PrEP. Daily PrEP is one of the standard ways of taking the drug, especially if you’re at risk of contracting HIV. It takes at least a week, or 7 days, before PrEP is effective for preventing HIV from receptive anal sex (bottoming). It takes 3 weeks, or 21 days, for PrEP treatment to be effective for preventing HIV from receptive vaginal sex and drug injections. When taken correctly, daily PrEP dosing is 99% effective for HIV prevention.
For many people, taking a daily dose every day is not an ideal regimen for a number of reasons. Some PrEP users might have trouble remembering to take their medicine every day, they might prefer more flexible dosing, or they’re not engaging in risky activities on a regular basis. Fortunately, event based PrEP is a more flexible way to take PrEP that doesn’t involve having to take a pill every day.
What is event-based PrEP?
Event-based PrEP, or on-demand PrEP, is an alternative way to take PrEP. It involves taking a PrEP pill 2 to 24 hours before engaging in sexual activity. A second pill is taking 24 hours after the first pill. Then, a third pill is taken 24 hours after the second pill. This method of taking PrEP is also sometimes called PrEP 2-1-1.
With event-based PrEP, you don’t have to worry about daily dosing. Instead, you can take PrEP as needed, which may be preferred if you have sex a limited amount of times per week or you can predict when you will have sex. You can even switch to taking PrEP this way instead of taking PrEP daily. There may be a number of reasons a person may want to take PrEP on demand. You might choose to try event-based PrEP if you are:
- A cisgender male who has receptive or insertive anal sex (bottom or top)
- Not engaging in risky sexual activity daily
- Having trouble taking a daily pill (because of forgetfulness or other problem)
- Preferring to have more flexibility with dosing
- Having concerns about side effects from long-term PrEP use
- Not using any type of injection drug use
- Having concerns about the potential high cost of daily PrEP use
Event-based PrEP may not be right for you if:
- You currently have an active hepatitis B infection
- It is hard to predict when you will be engaging in risky activities
- You are an injection drug user or sharing needles with HIV positive people
How effective is event-based PrEP?
Studies have shown that event-based PrEP is just as effective as daily PrEP dosing. According to the Ipergay study, taking PrEP on demand reduces the risk of getting HIV by 86% in men who have sex with men. The UK PROUD study showed an identical effectiveness of 86% for preventing HIV infection with a daily PrEP dosage. When taken appropriately, it is effective as a form of as-needed HIV protection.
Event-based PrEP is also well-tolerated with a low incidence of side effects. The most common side effects associated with on-demand PrEP are gastrointestinal effects like nausea, vomiting, and diarrhea. Studies show that nausea is twice as common in those who take event-based PrEP versus daily PrEP dosing.
Event-based PrEP is currently recommended as a dosing strategy for cisgender men only. It is not recommended for cisgender or transgender women. PrEP is absorbed into anal tissue more quickly than it is absorbed into vaginal tissue, and it may not be as effective in an event-based regimen for women. Not enough studies have been conducted on the effectiveness of on-demand PrEP in people other than cisgender men, and for that reason, cisgender and transgender women should take daily PrEP.
While on-demand PrEP is not FDA-approved, it has received widespread support. Currently, several organizations across the world endorse the use of PrEP on demand when deemed appropriate. The World Health Organization, European AIDS Clinical Society, British HIV Association, and International AIDS Society-USA all support the use of event-based PrEP.
Studies evaluating event-based PrEP included the use of Truvada. Not enough studies have been conducted to assess the use of Descovy for event-based PrEP.
How long is PrEP effective after stopping?
It can take around 20 days of daily dosing for PrEP to reach steady state levels in the body. Steady state refers to drug levels remaining consistent enough in the body to provide the maximum benefits. For men having sex with men, PrEP can provide maximum protection after 7 days.
PrEP may be effective for up to 7 days after stopping treatment with daily PrEP dosage strategy. Tenofovir disoproxil fumarate has a half-life of around 17 hours in the blood and 60 hours within the cells. The active component of emtricitabine has a half-life of around 39 hours.
When it comes to using PrEP on demand, you may only be protected for as long as a course of event-based PrEP is used. For this reason, this method is flexible but only effective when used properly. If you believe you’ll be engaging in risky activities regularly after stopping a course of event-based PrEP, you may be a candidate for taking daily PrEP. Or, you can take PrEP every day for as long as you are having sex, and then take one pill every day for two days more after the last day you had sex.
If you believe you’ve been exposed to HIV, you should continue to take PrEP for 28 days after the day of exposure. This would be similar to a dosing regimen called PEP, or post-exposure prophylaxis.
Event-based PrEP dosage
PrEP can be taken on an on-demand basis, especially if you can predict when you’ll be having sex. You may know ahead of time that you’ll need it on a certain day or for a weekend, for example. A standard dosage plan for event-based PrEP includes four pills for a single day of sex. You would use the following instructions:
- Take an initial dose of two pills between 2 and 24 hours before you’ll be having sex. It’s important to make sure you have at least 2 hours after taking the initial dose before sex.
- 24 hours after taking the initial dose, take a single pill.
- Then, take another pill 24 hours after that.
If you think you’ll be having sex for longer than just one day, you can take a pill every day you’re having sex. Just remember to take a dose for the next two days after the last day you stopped having sex.
If you’ll be having sex within 7 days after stopping a course of event-based PrEP, you can start another course. This time, however, you can take one pill instead of two pills as an initial dose 2 to 24 hours before sex. You would then take a single dose every day for 2 days after stopping sex.
If you did not have sex after taking the initial dose, you can keep taking a single a dose every day if you believe you’ll be having sex. Then, you can follow the plan for a normal course.
Other tips if you take PrEP on demand
Use an alarm and/or pillbox
While on-demand PrEP is more flexible than a daily dosing, there are still some things to keep in mind. It’s important that you take every dose on time. That might mean setting an alarm for when it’s time to take your next doses. You could also try using a pillbox to keep track of doses, especially if you’ll be having sex on and off. Once you take a dose, you can write a note in your phone or on a sheet of paper to keep track.
If you know you’ll be traveling or going out for the weekend, you might want to take spare doses with you just in case. You can also adjust your PrEP dosage depending on how your schedule plays out or if there are any changes. For example, if you take your initial dose on a Thursday night and you’re going out on the upcoming Saturday night, you can take your dose before you leave the house that night.
Splitting up the initial dose is OK
If you find that taking two pills at a time is difficult for you, you can split up the initial dose into one pill at different times. For example, you can take the first pill at breakfast and the second pill around lunchtime. Just ensure that you have at least 2 hours after taking the second pill before having sex. Also, remember to take a dose 24 hours after the second pill of the initial dose.
Missing a single dose of PrEP
It is important that you remember to take all doses of event-based PrEP. If the timing of a dose gets interrupted, just take the next dose as soon as you remember. You can also contact your sexual health clinic or doctor for further medical advice on what to do. In some cases, you may have to take PEP.
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