PrEP (pre-exposure prophylaxis) is a medication that HIV-negative people take to prevent HIV infection. When taken as prescribed, it reduces the risk of getting HIV from sex by about 99%. It works by keeping enough antiviral medication in your body that if HIV enters, the virus can’t establish an infection.
How PrEP Blocks HIV at the Cellular Level
HIV reproduces by hijacking your cells. Once inside a cell, the virus uses an enzyme called reverse transcriptase to copy its genetic material into DNA, which then gets woven into your cell’s own DNA. From that point, the cell becomes a factory for producing more virus.
PrEP medications interrupt this process at the copying step. The active drugs are structurally similar to the building blocks your cells use to construct DNA, so the virus’s enzyme grabs them during replication. But these imitation building blocks are defective: they’re missing a chemical component needed to attach the next piece. Once incorporated, the growing DNA chain hits a dead end. The virus can’t finish copying itself, so infection never takes hold.
The key is that PrEP needs to already be present in your tissues before exposure. If the drug is waiting in your cells when HIV arrives, the virus gets stopped before it can ever integrate into your DNA. This is why consistent dosing matters so much.
PrEP Options: Pills and Injections
There are currently three FDA-approved PrEP options, and they fall into two categories: daily pills and a long-acting injection.
- Truvada (tenofovir disoproxil fumarate/emtricitabine): A daily pill approved for all adults and adolescents weighing at least 77 pounds. It protects against HIV from sex and from injection drug use.
- Descovy (tenofovir alafenamide/emtricitabine): A daily pill with a newer formulation of tenofovir that is gentler on the kidneys and bones. It is not approved for people at risk through receptive vaginal sex, as it hasn’t been studied in that population.
- Apretude (cabotegravir): A long-acting injection given by a healthcare provider. After two initial shots one month apart, you receive an injection every two months. It works through a different mechanism, blocking a different enzyme (integrase) that HIV needs to insert its DNA into your cells. It’s approved for people 12 and older weighing at least 77 pounds.
The injectable option is particularly useful if taking a daily pill feels burdensome or hard to keep up with. Since a provider administers it on a set schedule, adherence becomes simpler.
How Long It Takes to Reach Full Protection
PrEP pills don’t provide maximum protection the moment you swallow the first dose. The medication needs time to build up to effective levels in different body tissues, and those timelines vary.
For receptive anal sex, PrEP pills reach maximum protection at about 7 days of daily use. For receptive vaginal sex and for people who inject drugs, it takes longer: about 21 days of daily use to reach full protection. The difference comes down to how quickly different tissues absorb and concentrate the drug. Rectal tissue takes up the medication faster and at higher concentrations than vaginal tissue or blood.
During this ramp-up period, you’re not unprotected, but you haven’t reached the 99% effectiveness level yet. Using condoms during this window adds an extra layer of safety.
What Happens Before You Start
Before prescribing PrEP, your provider will run several tests. The most important is an HIV test using a blood draw (not an oral rapid test, which is less sensitive and can miss recent infections). PrEP is only for people who are HIV-negative. Taking PrEP while unknowingly HIV-positive could lead to drug resistance, making the virus harder to treat later.
If you’re starting an oral pill, you’ll also need a kidney function test, since both pill formulations can affect the kidneys. Screening for hepatitis B is required as well, not because hepatitis B prevents you from using PrEP, but because the medications also have activity against that virus. Stopping PrEP abruptly if you have hepatitis B could cause a flare.
Your provider will also screen for other sexually transmitted infections like chlamydia, gonorrhea, and syphilis. For the injectable option, kidney testing isn’t necessary since cabotegravir doesn’t affect the kidneys the same way the oral medications do.
Effectiveness by the Numbers
PrEP reduces the risk of getting HIV from sex by about 99% when taken consistently. Among people who inject drugs, the risk reduction is at least 74%. That gap isn’t because the drug works differently in the bloodstream. It reflects the real-world difficulty of perfect daily adherence in populations facing additional challenges.
The word “consistently” is doing a lot of work in that statistic. Missing doses drops effectiveness significantly. Studies have shown that people who take PrEP four or more days per week still get strong protection, but skipping multiple days in a row opens real gaps. The injectable form sidesteps this problem by removing the daily decision entirely.
Common Side Effects
Most people tolerate PrEP well, and side effects that do occur tend to show up in the first few weeks and then fade. The most common ones include nausea, headache, dizziness, bloating, diarrhea, and trouble sleeping. These affect more than 1 in 100 people but are generally mild enough that most users continue treatment.
The more significant concern with oral PrEP is its potential effect on the kidneys. This is why kidney function is checked before starting and monitored periodically while you’re on it. If kidney function dips, your provider may switch you to a different formulation or stop PrEP. Bone density loss is another possible effect, though it’s rare and tends to reverse after stopping the medication. Descovy’s newer formulation of tenofovir was specifically designed to reduce both of these risks compared to Truvada.
The injectable form has its own side effect profile. Injection site reactions, including pain, swelling, or hardness at the injection site, are common, especially after the first few shots.
Ongoing Monitoring While on PrEP
PrEP isn’t a set-it-and-forget-it medication. You’ll need regular follow-up visits, typically every three months. At each visit, you’ll be tested for HIV again to confirm you’re still negative. STI screening is also repeated regularly, and kidney function is rechecked for those on oral PrEP.
These check-ins serve a dual purpose. They catch any breakthrough infections early (extremely rare with good adherence, but possible) and they monitor for side effects before they become serious. For injectable PrEP, these visits naturally coincide with your injection schedule.