Post-Exposure Prophylaxis (PEP) is an emergency medical treatment designed to prevent HIV infection after potential exposure to the virus. PEP involves taking a combination of antiretroviral medications, the same drugs used to treat HIV, for a short period. This measure works by interfering with the virus’s ability to establish a permanent infection. For PEP to be effective, it must be started as quickly as possible, and no later than 72 hours after the potential exposure.
The Length of the Standard PEP Treatment Course
The standard regimen for Post-Exposure Prophylaxis is a fixed 28-day course of daily medication. This means the patient takes the combination of antiretroviral drugs every day for four full weeks.
This 28-day timeline is the minimum duration required to ensure the medication concentration in the bloodstream is high enough to halt the initial replication of the human immunodeficiency virus. HIV establishes infection very quickly, often within the first 24 to 72 hours after exposure. Consistent adherence to the full course is necessary for the best chance of preventing infection.
If an individual stops taking the medication early, the drug levels may drop below the therapeutic concentration needed to suppress the virus. This could allow any virus that has entered the body to begin replicating and establish a permanent infection. The 28-day period is a therapeutic window where the drugs must be present to prevent the virus from taking hold.
Drug Metabolism and Clearance Post-Treatment
Once the 28-day treatment course is completed, the active compounds begin to clear from the body. The drugs used in modern PEP regimens, such as tenofovir, emtricitabine, and dolutegravir, are metabolized and eliminated at different rates. Clearance speed is primarily determined by the drug’s half-life, which is the time it takes for the concentration in the bloodstream to be reduced by half.
For nucleoside reverse transcriptase inhibitors, like emtricitabine, plasma levels drop below therapeutic concentration within a few days after the last dose. Tenofovir has a longer half-life in the blood, but its active form is concentrated inside cells where it performs its function.
Dolutegravir, a common third drug in a PEP regimen, has a relatively long half-life, but its plasma concentration falls below a minimum effective level in about 49 hours. While trace amounts of metabolites may be detectable longer, the drugs lose their therapeutic effect quickly once daily dosing stops. The active, infection-blocking concentration of the medication is generally cleared from the body within one week of taking the final pill.
Follow-Up Testing and Monitoring Schedule
The clinical timeline for confirming HIV status is often what people mean by how long PEP “stays in the system.” The presence of antiretroviral drugs can delay the body’s natural production of HIV antibodies, which determines the required waiting period for definitive testing. This delay is known as the window period.
The standard monitoring schedule begins with a baseline HIV test performed before the first dose of PEP is taken. A second HIV test is then performed immediately after the 28-day regimen is completed to check for early signs of infection. These early tests utilize fourth-generation assays, which detect both HIV antibodies and the p24 antigen, allowing for earlier detection than older antibody-only tests.
The most important test is the final, definitive test, which is typically scheduled at 4 to 6 weeks, or sometimes 12 weeks (3 months), after the initial exposure. The 12-week time frame is a conservative approach that provides high confidence in the result because it accounts for the maximum possible delay in antibody development due to the PEP medication. Receiving a negative result at the 12-week mark is generally considered conclusive proof that the PEP course successfully prevented HIV infection.
The final monitoring appointment also includes checking for potential side effects and performing other laboratory tests to ensure the overall health of the liver and kidneys was not affected by the medication. The entire process of taking the medication and completing the necessary follow-up testing takes approximately three months to resolve fully.