What to do about a missed dose depends on whether you are taking PrEP daily or following the event-driven (or on-demand) schedule. Read the appropriate section below for details.
TIPS! If you have trouble remembering to take your doses on-time, these tips may be helpful.
First of all, don't worry. There is solid research showing that "After 30 days of daily dosing, the estimated protection offered by the PrEP remaining in the body ... 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." If you've been on PrEP for at least 30 days, you are very well protected even if you missed several doses.
That said, if you have missed a dose, here's what to do:
If you are less than 12 hours late with your dose, go ahead and take it now, and then take your next dose at the regular time. If you are more than 12 hours late, skip the dose you forgot, and then take your next dose at the regular time. Missing one dose does not meaningfully reduce the protection you get from daily PrEP.
Example 1: Let’s say you take your Truvada every night at 10 PM. One morning you wake up at 8:00 and realize you forgot to take your dose last night. Because you’re less than 12 hours late, go ahead and take a pill now, and then take your next pill as usual at 10 PM that night.
Example 2: Let’s say you take your Truvada every night at 10 PM. One day, at 3 o’clock in the afternoon, you realize you forgot to take your dose last night. Because you’re more than 12 hours late, skip the dose you missed, and take your next pill as usual at 10 PM that night.
Once you have been taking Truvada daily for 30 days, the active drugs have built up in your body and are not eliminated quickly. Even when you miss doses, the remaining drug in your body continues to protect you from HIV. If you miss doses or stop taking the medication, the accumulated drug in your body continues to reduce your risk by 97% after 24 hours, 96% after 3 days, 93% after five days, and 90% after a full week. Note that these numbers apply only to exposures through anal sex. We do not yet know how PrEP’s protection builds up and decays for vaginal/front hole sex. If you have receptive vaginal/front hole sex, every dose matters.
If you are following the PrEP 2-1-1 schedule (only before and after sex), it is important that you take every required dose on schedule. There is no solid research yet about the effect of missed doses with event-driven PrEP, but missing doses is likely to reduce your level of protection from HIV.
You can help yourself remember to take your pills on schedule using a keychain pill reminder, a pill bottle with a built-in alarm, a 7-day pill box, or a reminder app for your smartphone. Here are some links that may help:
More ideas: