Pre-exposure prophylaxis (PrEP) is a medication taken by HIV-negative individuals to prevent HIV infection. This preventative measure is highly effective when taken as prescribed, but doses can occasionally be missed. PrEP works by maintaining a sufficient concentration of antiretroviral drugs in the body’s tissues, stopping the virus from replicating if exposure occurs. The margin of error for missed doses depends entirely on the specific dosing schedule and the consistency of adherence.
Understanding PrEP Dosing Schedules
The management of a missed dose depends on the two primary oral PrEP schedules: daily and event-driven dosing. Daily PrEP requires taking one tablet every 24 hours to maintain continuous drug levels in the body. This schedule is suitable for all individuals at risk, including those who have vaginal, frontal, or anal sex, or who inject drugs.
Event-driven dosing, also known as PrEP 2-1-1, is primarily recommended for cisgender men who have anal sex and can plan sexual activity in advance. This schedule involves taking two tablets 2 to 24 hours before sex, followed by one tablet 24 hours after the initial dose, and a final tablet 24 hours after the second dose. The timing elevates the drug level specifically around the time of potential exposure, making a missed dose a more immediate concern.
Guidance for Missing a Single Dose
Daily PrEP
If following a daily PrEP schedule, there is a forgiveness window for a single missed pill. If you realize you missed a dose, take it immediately, provided it is within 12 hours of your usual time. Taking the dose within this timeframe allows you to stay on track and resume your regular schedule the next day.
If more than 12 hours have passed, skip the missed pill entirely and take your next dose at the usual time. Do not double the dose to make up for the missed one, as this increases the risk of side effects without providing additional protective benefit. For those consistently taking daily PrEP, missing a single dose is unlikely to compromise overall protection, especially if at least six doses were taken in the past week.
Event-Driven PrEP
Guidance for event-driven PrEP is less flexible because the regimen creates short-term, peak drug concentrations around a specific event. Missing any dose in the 2-1-1 sequence reduces protective drug levels when they are needed most. If you miss or are late with the initial double dose, take the two tablets as soon as possible before sex, or immediately after, and then continue the rest of the 2-1-1 sequence.
A lapse in any event-driven dose should prompt a conversation with a healthcare provider to assess whether emergency post-exposure prophylaxis (PEP) is needed. The margin of error is smaller since protection is not built up over weeks, unlike the daily schedule.
Re-establishing Protection After Multiple Missed Doses
The risk of HIV acquisition increases when multiple daily PrEP doses are missed, causing drug concentrations to fall below the protective threshold. For anal sex, taking fewer than four pills a week may not provide adequate protection. Those at risk through vaginal or frontal sex require consistent daily use for full protection.
If you stop taking PrEP for a week or more, drug levels drop, meaning you are no longer protected and must restart the regimen. If you have missed several doses or stopped PrEP, do not restart the daily regimen on your own. Contact your healthcare provider immediately for guidance.
Before re-initiating PrEP, getting re-tested for HIV is crucial, regardless of potential exposure. Starting PrEP with an undiagnosed HIV infection can lead to the virus developing resistance, limiting future treatment options. Your provider will perform the HIV test and may recommend alternative prevention methods, such as condoms, until a protective drug level is re-established. Depending on the type of sex, this re-loading period can take up to seven consecutive days of dosing before full protection is achieved.