Pre-Exposure Prophylaxis (PrEP) is a medication designed to prevent Human Immunodeficiency Virus (HIV) infection. It is for individuals who do not have HIV but are at ongoing risk of exposure, such as through sexual activity or injection drug use. PrEP works by blocking an enzyme HIV needs to replicate, stopping the virus from establishing an infection. When taken as prescribed, PrEP helps the body produce protective medication levels, significantly lowering the chance of acquiring HIV.
Understanding PrEP’s Presence in the Body
The duration PrEP remains active in the body is understood through pharmacokinetics, which describes how a drug moves through and is processed. Key concepts include “half-life” and “steady-state.” A drug’s half-life is the time it takes for its concentration in the bloodstream to reduce by half. For oral PrEP components, emtricitabine has a plasma half-life of approximately 10 hours, and tenofovir disoproxil fumarate about 17 hours.
The active form of tenofovir, tenofovir diphosphate, has a longer half-life within cells (3-4 days). Achieving “steady-state” means the drug entering the body with each dose equals the amount eliminated, resulting in consistent levels. This balance is typically reached after four to five half-lives. For PrEP, stable levels ensure the medication is continuously present in tissues where HIV exposure might occur, providing ongoing protection.
Factors Influencing PrEP’s Duration
While PrEP generally follows predictable absorption and elimination patterns, individual biological differences can affect how long it remains at protective levels. Kidney function plays a role, as PrEP components are primarily cleared by the kidneys. Individuals with reduced kidney function, including older individuals, may process the medication more slowly, potentially leading to higher drug levels or longer elimination times. Consistent adherence to the prescribed dosing schedule also significantly influences drug duration and effectiveness. Missing doses can lead to fluctuating drug levels, potentially falling below the threshold for optimal protection.
Maintaining Protection with PrEP
Understanding how long PrEP remains in the body is directly linked to maintaining consistent protection against HIV. Daily or consistent dosing is necessary to ensure drug levels remain above the protective threshold. If doses are missed, drug levels may decrease, potentially reducing protection.
While missing a single dose may not significantly impact protection due to the active drug’s longer half-life within cells, consistently missing multiple doses can compromise efficacy. Protection may wane within 7 to 10 days after discontinuing daily oral PrEP. Therefore, adherence is emphasized to ensure the medication provides its full preventive benefit.
Different PrEP Formulations and Their Duration
Currently, there are two main oral PrEP formulations: emtricitabine/tenofovir disoproxil fumarate (often known by its brand name Truvada) and emtricitabine/tenofovir alafenamide (brand name Descovy). Truvada is approved for all people at risk of HIV through sex or injection drug use. Descovy is approved for sexually active cisgender men and transgender women at risk of HIV, but it is not indicated for individuals at risk through receptive vaginal sex.
The time it takes for these medications to reach protective levels varies depending on the type of exposure. For receptive anal sex, oral PrEP generally reaches maximum protection after about 7 days of daily use. For receptive vaginal sex and injection drug use, oral PrEP takes approximately 21 days of daily use to reach maximum protective levels. The tenofovir alafenamide component in Descovy has a plasma half-life of about 0.51 hours, while its active metabolite, tenofovir, has a median half-life of 32.37 hours. These differences in pharmacokinetic profiles contribute to the specific recommendations for each formulation’s use and time to effectiveness.