Pre-Exposure Prophylaxis (PrEP) is a daily medication taken by HIV-negative individuals to prevent HIV infection. The regimen involves taking one pill containing a combination of two antiretroviral drugs. This builds protective levels in the body, ensuring that if a person is exposed to HIV, the medication is present to stop the virus from establishing a permanent infection. Consistent daily adherence is necessary for PrEP to be highly effective.
Establishing the PrEP Baseline
The effectiveness of PrEP depends on maintaining a therapeutic concentration of the drugs in the tissues where HIV exposure is most likely to occur. This baseline of protection takes time to establish after starting the medication. For individuals engaging in receptive anal sex, highly protective levels are typically reached in the rectal tissue after about seven consecutive days of daily dosing.
The time needed to establish protection varies depending on the type of exposure. For those at risk through receptive vaginal sex, maximum drug concentrations in cervicovaginal tissue may take up to 20 days of daily dosing to fully develop. Medications like FTC/TDF and FTC/TAF require this consistency, as the drug reservoir established in the tissue provides the protective barrier against the virus.
If You Missed Only One Dose
Missing a single dose of daily PrEP is the most common scenario and generally does not significantly compromise the established level of protection. If a person realizes they missed a dose, they should take the forgotten pill as soon as possible to restore drug concentration with minimal disruption.
Never take a double dose to compensate for the missed pill, as this increases the risk of side effects without providing additional protective benefit. If the time for the next scheduled dose is approaching, the missed dose should simply be skipped. The person should then return to their regular daily schedule, taking the next pill at its usual time.
If You Missed Multiple Doses
Missing multiple consecutive doses significantly reduces the drug concentration in the body and compromises protection. Missing two or more doses can cause therapeutic drug levels to drop below the threshold necessary to block HIV infection effectively. At this point, the individual should not simply resume the regimen without guidance from a medical professional.
If multiple doses have been missed, the person must contact their healthcare provider immediately for consultation. The provider will need to assess the level of risk. Before re-initiating the daily medication, a new HIV test is mandatory to confirm the person is still HIV-negative. This step is necessary because the lapse in adherence means a recent HIV exposure could have resulted in infection, requiring an entirely different treatment regimen.
Follow-Up and Re-Establishing Adherence
After a lapse in adherence, the next step involves follow-up care and planning. The healthcare provider will likely recommend an accelerated schedule for follow-up HIV testing, rather than waiting for the standard three-month check. This proactive testing is important because the risk of HIV acquisition is highest during periods when adherence is poor.
Open communication with the medical team is necessary to identify the factors that led to the missed doses. The provider can help address barriers such as scheduling difficulties or managing minor side effects. To prevent future lapses, they can help develop concrete strategies for consistent adherence, such as using pill boxes, setting smartphone alarms, or linking the pill-taking to a daily routine.