Pre-exposure prophylaxis, widely known as PrEP, is a highly effective medical intervention designed to prevent the acquisition of the Human Immunodeficiency Virus (HIV). This strategy involves an HIV-negative person taking specific antiretroviral medication before potential exposure to the virus. PrEP is approved for use by individuals of all genders, including those who engage in heterosexual sex and are at risk of acquiring HIV. It works by establishing protective drug levels in the body to stop the virus from taking hold if exposure occurs.
Determining Eligibility Based on Heterosexual Risk
Determining eligibility for PrEP centers on a thorough assessment of an individual’s potential for HIV exposure through sexual activity or injection drug use. The primary focus for heterosexual individuals is whether they have had vaginal or anal sex within the past six months and meet specific risk criteria. One of the clearest indications for PrEP is having a sexual partner who is known to be living with HIV, especially if that partner is not consistently taking their own medication or has a detectable viral load. This scenario is often referred to as being in a serodifferent relationship.
Having sex without a condom consistently with partners whose HIV status is unknown places a person at increased risk. A recent diagnosis of a bacterial sexually transmitted infection (STI) like syphilis, gonorrhea, or chlamydia within the last six months is another indicator of elevated risk. The presence of an STI suggests a biological environment that may make HIV acquisition easier, alongside an indication of higher-risk sexual practices. Individuals who engage in transactional sex, such as exchanging sex for money, shelter, or drugs, are also recognized as being at a substantial risk for HIV acquisition.
How PrEP Works and Its Effectiveness for Sexual Transmission
The effectiveness of PrEP stems from its biological mechanism, which targets the machinery HIV needs to replicate within the human body. The medication consists of a combination of two antiviral compounds that belong to a class of drugs called nucleoside/nucleotide reverse transcriptase inhibitors. These compounds, specifically tenofovir and emtricitabine, are absorbed into the bloodstream and sexual tissues, including the vaginal and rectal lining.
When HIV enters the body, it attempts to infect immune cells by converting its genetic material from RNA into DNA, a process mediated by a viral enzyme called reverse transcriptase. The PrEP medication blocks this enzyme and prevents the virus from completing this conversion. By stopping the virus from integrating its genetic code into the host cell’s DNA, the medication effectively prevents the infection from establishing itself and spreading throughout the body.
Consistent daily use is directly linked to maximum protection, as it maintains sufficient drug concentrations to neutralize the virus. When taken as prescribed, oral PrEP reduces the risk of acquiring HIV through sexual contact by approximately 99%. For individuals engaging in receptive vaginal sex, it takes about 21 days of daily pill use to reach the maximum protective level in the vaginal tissues. The time required for full protection emphasizes that PrEP is a prophylactic measure requiring planning and adherence, not a last-minute intervention.
Approved Formulations and Dosing Schedule
The United States Food and Drug Administration (FDA) has approved several formulations for oral PrEP, primarily consisting of two different drug combinations for daily use. The first is a fixed-dose combination of tenofovir disoproxil fumarate and emtricitabine (TDF/FTC), which is approved for all individuals at risk of sexually acquired HIV. The second oral option is a combination of tenofovir alafenamide and emtricitabine (TAF/FTC).
The standard recommendation for heterosexual individuals is to take one tablet of the prescribed PrEP medication once daily. This regimen is necessary to ensure the drug reaches and maintains therapeutic concentrations in the vaginal tissue, which is a common site of exposure in heterosexual sex. Alternative dosing strategies, such as “on-demand” or “2-1-1” dosing, have not been studied or recommended for individuals who may acquire HIV through receptive vaginal sex.
The TAF/FTC formulation is not approved for individuals who are at risk of acquiring HIV through receptive vaginal sex. This is because clinical trials did not demonstrate sufficient drug levels in vaginal tissue for protection. Therefore, individuals engaging in receptive vaginal sex must be prescribed the TDF/FTC combination.
Starting PrEP: Required Testing and Safety Profile
Before initiating PrEP, a healthcare provider must confirm that the individual is HIV-negative. Starting PrEP while having an undiagnosed HIV infection can lead to the virus developing resistance to the medication. This initial screening requires a highly accurate HIV test, typically a fourth-generation antigen/antibody test.
Baseline testing also includes screening for Hepatitis B status, as the PrEP medication components have activity against this virus. Furthermore, kidney function is assessed by measuring blood serum creatinine to estimate creatinine clearance. The TDF/FTC formulation can potentially affect kidney function in rare cases, making this monitoring a necessary safety precaution.
Once PrEP is started, individuals are required to have follow-up visits with their provider, usually every three months, for repeat HIV testing and prescription refills. These periodic check-ups involve monitoring for side effects, which are generally mild and temporary (e.g., headache, nausea, or stomach upset). Less common side effects associated with the TDF/FTC formulation can include minor changes in bone mineral density or kidney function, which are tracked through routine lab work.