HIV PrEP, short for pre-exposure prophylaxis, is a medication strategy that prevents HIV infection in people who do not yet have the virus but face a meaningful risk of acquiring it. When taken consistently, oral PrEP reduces the risk of getting HIV through sex by more than 90 percent. The approach works by maintaining protective levels of antiretroviral drugs in the body’s tissues so that if HIV is encountered, the virus cannot establish a foothold. What started as a once-daily pill has expanded into injectable options given every two months or even twice a year, reshaping how prevention fits into people’s lives.
How PrEP Blocks HIV at the Cellular Level
The original and still most widely used PrEP regimen combines two drugs: tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC). Both belong to a class of antiretrovirals that interfere with reverse transcriptase, the enzyme HIV needs to copy its genetic material into a human cell’s DNA. Once you swallow the pill, these drugs are absorbed into your bloodstream and taken up by cells in tissues where HIV typically enters the body, particularly mucosal tissue in the rectum, vagina, and cervix. Inside those cells, the drugs undergo a chemical activation process, converting into their active forms (diphosphate and triphosphate molecules) that can intercept HIV’s replication machinery.1PubMed Central. Pre-exposure Prophylaxis (PrEP) for HIV Infection: How Antiretroviral Pharmacology helps to Monitor and Improve Adherence If HIV tries to reverse-transcribe its RNA into DNA, the active drug molecules get incorporated into the growing DNA chain and stop it from being completed. No completed DNA copy means no infection.
One detail that matters for different types of exposure is that these drugs do not reach all tissues equally. Tenofovir concentrates heavily in rectal tissue, where levels remain detectable for about two weeks after a single dose and run roughly a hundred times higher than in vaginal or cervical tissue.2PubMed Central. Penetration of tenofovir and emtricitabine in mucosal tissues: implications for prevention of HIV-1 transmission Emtricitabine, by contrast, reaches higher concentrations in vaginal and cervical tissue but clears from all tissue types faster. This uneven distribution is one reason protection during receptive anal sex may build up within a few days of starting PrEP, while full protection during vaginal sex is thought to require about seven days of daily dosing. It also informs why certain dosing strategies work better for certain exposure routes.
Available Formulations and Dosing Options
There are now several ways to take PrEP, each suited to different lifestyles and preferences.
- Daily oral TDF/FTC (Truvada and generics): One pill every day. This is the most studied formulation and the one with the longest track record. Generic versions have made it the least expensive oral option.
- Daily oral TAF/FTC (Descovy): A newer pill that uses tenofovir alafenamide instead of tenofovir disoproxil fumarate. A large trial found it was equally effective for HIV prevention and showed better markers for bone density and kidney function compared with TDF/FTC.3The Lancet HIV. Once-daily emtricitabine and tenofovir alafenamide versus emtricitabine and tenofovir disoproxil fumarate for pre-exposure prophylaxis of HIV (DISCOVER) It is approved for men and transgender women who have sex with men but has not been studied for vaginal exposure, so it is not currently recommended for cisgender women.
- On-demand (2-1-1) dosing: Instead of daily pills, you take a double dose of TDF/FTC two to 24 hours before sex, then one pill 24 hours after the first dose and another 48 hours after the first dose. This approach is supported by the IPERGAY trial and endorsed by some international guidelines. Among people who switched to this schedule, the most common motivations were having sex less frequently and wanting to take fewer pills.4PubMed Central. Facilitators and barriers of 2-1-1 HIV pre-exposure prophylaxis The main challenge is unplanned sex, which can mean missing that crucial pre-sex double dose. This regimen is validated only for anal exposure, not vaginal.
- Long-acting injectable cabotegravir (Apretude): An injection given every two months after two initial monthly shots. More on this below.
A survey of PrEP users who switched from TDF/FTC to TAF/FTC found that over half did so on a doctor’s recommendation, while about a third cited perceived improved safety as their own reason for switching.5PubMed Central. Why Are Patients Switching from Tenofovir Disoproxil Fumarate/Emtricitabine (Truvada) to Tenofovir Alafenamide/Emtricitabine (Descovy) for Pre-Exposure Prophylaxis? Whether switching makes clinical sense depends on your individual kidney and bone health profile, something worth discussing with your provider rather than assuming newer automatically means better.
Who Should Consider PrEP
PrEP is not designed for everyone. It is specifically for HIV-negative people whose circumstances put them at substantial risk of acquiring the virus. The U.S. Preventive Services Task Force (USPSTF) recommends PrEP for people who meet any of the following profiles: men who have sex with men and have a partner living with HIV, inconsistent condom use during anal sex, or a recent bacterial sexually transmitted infection; heterosexually active people who have a partner with HIV, inconsistent condom use with a high-risk partner, or a recent STI; and people who inject drugs and share injection equipment or have sexual risk factors.6JAMA. Preexposure Prophylaxis for the Prevention of HIV Infection: US Preventive Services Task Force Recommendation Statement Additional clinical indicators include having a high number of sex partners and engaging in commercial sex work.7Clinical Infectious Diseases. US Guideline Criteria for Human Immunodeficiency Virus Preexposure Prophylaxis: Clinical Considerations and Caveats
An important prerequisite is a documented negative HIV test before starting PrEP. Taking PrEP when you already have HIV, even unknowingly during acute infection, can lead to drug resistance mutations. One study of PrEP starts found that among a small number of people who turned out to have undiagnosed acute HIV when they began, about a quarter developed a resistance mutation within days, though all achieved viral suppression once they were linked to full HIV treatment.8JAIDS Journal of Acquired Immune Deficiency Syndromes. Acute HIV at the Time of Initiation of Pre-exposure or Post-exposure Prophylaxis: Impact on Drug Resistance and Clinical Outcomes This is why HIV testing right before starting PrEP is standard practice, not an optional formality.
PrEP for People Who Inject Drugs
PrEP is recommended for people who inject drugs, yet uptake in this group remains strikingly low. A qualitative study found that while interest in PrEP was high among people who inject drugs, barriers operated at multiple levels, from lack of awareness and unstable housing to providers not offering it and the stigma of seeking HIV prevention.9PubMed Central. Perspectives on HIV pre-exposure prophylaxis (PrEP) utilization and related intervention needs among people who inject drugs Cost-effectiveness analyses suggest that enrolling all people who inject drugs indiscriminately is expensive relative to targeting those with the most HIV-positive needle-sharing partners, where the cost per quality-adjusted life year becomes more favorable.10PubMed Central. Cost-effectiveness of alternative strategies for provision of HIV preexposure prophylaxis for people who inject drugs The gap between clinical recommendation and real-world access is wider for this group than for almost any other population PrEP is meant to serve.
PrEP During Pregnancy and Breastfeeding
People who become pregnant while on PrEP, or who are at risk of HIV during pregnancy, face a distinct set of questions. A systematic review of safety data found no differences in pregnancy or birth outcomes linked to PrEP exposure, and described the early evidence as reassuring.11PubMed Central. Emerging evidence from a systematic review of safety of pre-exposure prophylaxis for pregnant and postpartum women: where are we now and where are we heading? A trial examining oral PrEP during breastfeeding reported no HIV infections among participants and low rates of serious adverse events in both mothers and infants.12The Lancet HIV. Safety and pharmacokinetics of the dapivirine vaginal ring and oral pre-exposure prophylaxis during breastfeeding (MTN-043) HIV acquisition during pregnancy or breastfeeding carries an especially high risk of mother-to-child transmission, so continuing or starting PrEP during these periods can be important for people with ongoing exposure risk.
How Effective Is PrEP in Practice
The gap between how well PrEP works in theory and how well it works in someone’s daily life comes down to one thing: whether the drug is actually in the body when exposure happens. The landmark trials showed risk reductions ranging from 44 percent in iPrEx (among men who have sex with men and transgender women, where adherence varied widely) to 75 percent in Partners PrEP (among serodiscordant couples in East Africa, where adherence was higher).13PubMed Central. HIV PrEP Trials: The Road to Success In every major trial, the people with detectable drug levels in their blood were almost completely protected.
Real-world data from France showed the same dose-response pattern: people with low PrEP consumption had only about 18 percent effectiveness, those with intermediate consumption had about 69 percent, and high consumers reached 93 percent effectiveness.14The Lancet Public Health. Effectiveness of pre-exposure prophylaxis for HIV infection in real-world practice in France Modeling work reinforces this: even the pattern of missed doses matters, not just the total number missed. Someone who takes pills in scattered bursts gets less coverage than someone who spaces their doses more evenly, even if the total number of pills taken is identical.15PubMed Central. PrEP adherence patterns strongly impact individual HIV risk and observed efficacy in randomized clinical trials The practical takeaway is that PrEP is extraordinarily effective when taken, and much less so when it isn’t. This is why newer long-acting options that reduce the burden of daily adherence matter so much.
Side Effects and Safety
The side effect profile of oral PrEP is mild for most people. A systematic review and meta-analysis of 13 randomized trials found no significant difference in the overall rate of adverse events between people taking TDF/FTC and those taking placebo. The analysis also found no significant difference in fracture rates, which served as a proxy for bone effects.16Journal of Virus Eradication. How safe is TDF/FTC as PrEP? A systematic review and meta-analysis of the risk of adverse events in 13 randomised trials of PrEP Some people experience a “start-up syndrome” of nausea, headache, or mild stomach upset in the first few weeks, which typically resolves on its own.
The main safety considerations with TDF-based PrEP involve kidney function and bone density. Small, reversible decreases in both have been documented during use, which is why guidelines call for an initial visit, a follow-up after one month, and check-ins every three months that include kidney monitoring and HIV re-testing.17PubMed Central. PrEP-RN: Clinical Considerations and Protocols for Nurse-Led PrEP TAF/FTC showed improved markers in both kidney and bone safety compared with TDF/FTC in a head-to-head trial, which is why some providers recommend it for people with pre-existing concerns in those areas.
Drug resistance is another worry people often raise. The evidence suggests that resistance developing from PrEP use is uncommon and almost exclusively occurs when someone starts PrEP while unknowingly in the acute phase of an HIV infection, before standard tests can detect the virus.18PubMed Central. Drug Resistance during HIV Preexposure Prophylaxis For people who are truly HIV-negative when they begin and stay on PrEP, resistance is not a realistic concern.
Long-Acting Injectable Cabotegravir
For people who struggle with daily pills or simply prefer not to take them, injectable cabotegravir (sold as Apretude) represents a fundamentally different approach. It works through a different mechanism than the oral drugs, targeting the integrase enzyme that HIV uses to insert its genetic material into human chromosomes. After two initial monthly injections, it is given as an intramuscular injection every two months.
The HPTN 083 trial compared cabotegravir injections to daily oral TDF/FTC in cisgender men and transgender women who have sex with men. HIV incidence was roughly 0.4 per 100 person-years in the cabotegravir group versus about 1.2 per 100 person-years in the oral PrEP group, translating to about a 66 percent lower risk of infection with the injectable.19PubMed Central. Cabotegravir for HIV Prevention in Cisgender Men and Transgender Women Extended follow-up into the open-label phase confirmed this advantage held, with nearly identical risk ratios.20The Lancet HIV. Extended safety and efficacy evaluation of long-acting injectable cabotegravir compared with daily oral tenofovir disoproxil fumarate plus emtricitabine for pre-exposure prophylaxis A companion trial (HPTN 084) found similar superiority in cisgender women.
The injectable’s advantage is not that the drug is inherently more potent against HIV. It’s that adherence is no longer dependent on daily decisions. You show up every two months, get a shot, and you’re covered. That said, cabotegravir’s cost remains substantially higher than generic oral PrEP. A cost-effectiveness analysis estimated that to meet standard value thresholds compared with generic TDF/FTC, cabotegravir would need to be priced at less than about $4,100 per year.21PubMed Central. Cost-Effectiveness of Long-Acting Injectable HIV Preexposure Prophylaxis in the United States Current pricing far exceeds that, making access dependent on insurance coverage and assistance programs.
Twice-Yearly Lenacapavir
The most dramatic development in PrEP is lenacapavir, a capsid inhibitor given as a subcutaneous injection just twice a year. It works by a mechanism distinct from all existing PrEP drugs, targeting the protein shell (capsid) that HIV needs to assemble and disassemble during its life cycle.22PubMed Central. Emerging concepts in HIV pre-exposure prophylaxis: focus on twice-yearly lenacapavir
Trial results have been striking. In cisgender women in sub-Saharan Africa, zero HIV infections occurred among over 2,100 participants receiving lenacapavir, compared with a background incidence of about 2.4 per 100 person-years in the screened population.23PubMed. Twice-Yearly Lenacapavir or Daily F/TAF for HIV Prevention in Cisgender Women In a separate trial among men and gender-diverse persons, only 2 HIV infections occurred among over 3,200 lenacapavir recipients, compared with 9 in the oral PrEP group, an 89 percent lower rate.24PubMed. Twice-Yearly Lenacapavir for HIV Prevention in Men and Gender-Diverse Persons Two injections per year essentially eliminates the adherence problem that has been oral PrEP’s Achilles’ heel. Lenacapavir received FDA approval for PrEP in late 2024, though pricing and access questions remain significant, particularly for the low-income countries where HIV incidence is highest.
PrEP and Sexually Transmitted Infections
One persistent question about PrEP is whether removing the fear of HIV leads people to take more sexual risks, resulting in more bacterial STIs like gonorrhea, chlamydia, and syphilis. The data here are genuinely mixed, and the reality is more nuanced than either side of the debate usually presents.
A Danish study found that STI rates among men who have sex with men rose after PrEP initiation, but the increase actually began 10 to 20 weeks before PrEP started, suggesting that the behavioral shift preceded PrEP rather than being caused by it.25PubMed Central. Questioning risk compensation: pre-exposure prophylaxis (PrEP) and sexually transmitted infections among men who have sex with men, capital region of Denmark, 2019 to 2022 A U.S. demonstration study found that PrEP users actually reported fewer partners whose HIV status was unknown and showed no consistent increase in condomless sex or STI rates compared with non-PrEP users.26PubMed Central. Sexual Risk Compensation in a Pre-exposure Prophylaxis Demonstration Study among Individuals at Risk for HIV
What is clearly true is that PrEP brings people into regular healthcare contact, meaning STIs get tested for and detected more often. Whether higher detection rates reflect genuinely more infections or just better surveillance has been debated for years. Regardless, the regular STI screening built into PrEP care creates an opportunity to catch and treat infections that might otherwise circulate undiagnosed.
An emerging companion strategy is doxycycline post-exposure prophylaxis, or doxy-PEP, where a single dose of the antibiotic doxycycline is taken within 72 hours after condomless sex. Real-world data from a PrEP cohort showed that doxy-PEP reduced syphilis incidence from about 6.4 to 2.0 per 100 person-years and chlamydia incidence from about 31.9 to 8.8 per 100 person-years.27Sexually Transmitted Infections. Trends of bacterial sexually transmitted infections with doxycycline post-exposure prophylaxis among people using pre-exposure prophylaxis for HIV The trade-off is a legitimate concern about promoting antibiotic resistance, which will require careful monitoring as doxy-PEP use expands.
Racial and Geographic Disparities in Access
PrEP’s effectiveness as a public health tool depends on whether the people most at risk can actually get it, and here the record is troubling. In Canada, a national study found that Black men who have sex with men were roughly one-third as likely to be aware of PrEP as white men, and significantly less likely to be using it even when eligible. Indigenous men also showed lower awareness, particularly those in rural communities.28PubMed Central. Racial disparities in HIV pre-exposure prophylaxis (PrEP) awareness and uptake among white, Black, and Indigenous men in Canada Similar patterns are well-documented in the United States.
This is not just an equity problem in the abstract. A modeling study projected that if PrEP coverage increased sixfold without addressing racial inequities in who receives it, the disparity in HIV incidence between Black and white men who have sex with men would actually widen. The model predicted a 24 percent reduction in HIV incidence for white men but only 10 percent for Black men under equal scale-up, because the underlying coverage gap meant the gains flowed disproportionately to those already better served.29PubMed Central. Effect of Racial Inequities in Pre-Exposure Prophylaxis Use on Racial Disparities in HIV Incidence Among Men Who Have Sex with Men: A Modeling Study Scaling up PrEP without deliberately addressing who gets access risks making HIV disparities worse, not better. That finding alone should inform how public health programs allocate resources for outreach, provider training, and cost subsidies.