Missing One Dose of PrEP 8 Days After Exposure?


Hello, I'm a bit confused regarding my PrEP - I have been consistent in taking my prep pill once a day, every morning for the last 4 months. However, 8 days ago I had insertive anal sex (me being the top) without a condom with another man (of unknown HIV status). This morning (8 days after exposure) I realized I have missed one dose - i.e. this morning I realized I forgot to take it yesterday morning. My question is - am I at (higher) risk of contracting HIV from my 8 days-ago-exposure?

I have taken my pill and will keep taking it every day for a few months still, but, in general, if I were to stop (intentionally or unintentionally) taking my PrEP medication, how long after the LAST exposure is it safe to stop taking it?

Thank you so much for this great site!




Hi L. - Thank you for writing in. Congratulations on starting PrEP and using it consistently for four months. That means that the medications in the drug have had an opportunity to build up in your system over a period of time (more than 30 days). Once Truvada is in your system for a period of time, it is considered to be a very "forgiving" drug. Meaning, even if you miss a dose on some occasions, the drug stays active and protective for at least 5 extra days.

According to the most recent science - after 30 days of daily dosing, the estimated protection offered by the PrEP remaining in the body from HIV transmission via anal intercourse was 97%, 24 hours after the time of the first missed dose, 96% after three days, 93% after five days, and was still 90% a week later. (http://www.aidsmap.com/Intermittent-PrEP-may-be-a-robust-strategy-for-anal-sex-vaginal-much-less-certain/page/2986809/).

The other thing to keep in mind is that no one in any of the efficacy trials acquired HIV when they used Truvada four or more times per week. So although daily use is recommended in the U.S., there is clearly still great benefit even when used less than that (https://www.robertmgrant.org/project/has-prep-ever-failed/).

This is good news for those who may not be 'perfect' in taking doses every day for weeks and months on end. One need not be perfect to have maximal protection, just consistent. This is one of the reasons why Truvada was the first drug to be studied for PrEP - researchers understood that someone who is not living with HIV may have difficulty taking a pill perfectly every day - and this drug doesn't have to be taken perfectly to work.

The other thing to keep in mind: Even if you weren't on PrEP at all, and if your partner had a detectable level of HIV, your possibility of acquiring HIV from from topping from one encounter is still less than 1% (https://www.poz.com/article/HIV-risk-25382-5829). Then when you think about reducing that 1% by another 99% with daily PrEP, you can see that that you were well protected during this encounter, and in the 8 days after.

I encourage you to be aware of staying consistent with PrEP use, but not to excessively worry or stress about it. To learn more about PrEP, efficacy, and read real-world experiences, please check out the international Facebook group at: https://www.facebook.com/groups/PrEPFacts/