Pre-exposure prophylaxis, commonly called PrEP, is a medication taken by people who don’t have HIV to prevent them from getting it. When taken as directed, PrEP reduces the risk of acquiring HIV by up to 99%. It’s available as a daily pill or as an injection given every two months, and most insurance plans are now required to cover it at no cost to the patient.
How PrEP Stops HIV
HIV works by hijacking your immune cells and using their machinery to copy itself. PrEP drugs get absorbed into your cells and sit there waiting. If HIV enters your body, it tries to convert its genetic material into a form your cells can read, a step it needs to complete before it can establish a permanent infection. The active ingredients in PrEP mimic the building blocks HIV needs for that conversion process, but they’re defective copies. When HIV grabs them instead of the real thing, the copying process stalls and the virus can’t replicate.
This is why PrEP needs to be in your system before exposure. The drug has to already be present inside the cells HIV targets. Once it’s there, your body essentially has a chemical barrier in place that blocks the virus at one of its earliest and most vulnerable steps.
Available Forms of PrEP
Three FDA-approved PrEP options exist today, and they fall into two categories: daily pills and a long-acting injection.
Daily Oral PrEP
The two oral options are Truvada and Descovy. Both contain two antiretroviral drugs in a single tablet taken once a day. Truvada was the first PrEP medication approved and is now available as a generic, which has made it significantly cheaper. Descovy, approved later, showed better markers for kidney function and bone density in clinical trials, making it a common choice for people with concerns about those areas. However, Descovy is only approved for PrEP in people at risk through sex, not through injection drug use, and it hasn’t been studied for vaginal exposure.
Injectable PrEP
Apretude is a long-acting injection given in the buttock muscle. The schedule starts with two injections one month apart, then continues with one injection every two months after that. In clinical trials, it was compared head-to-head against daily Truvada and proved highly effective. It’s approved for adults and adolescents weighing at least 77 pounds who are at risk through sex.
The injection can be a good fit if you find it hard to take a pill every day, or if you simply prefer a shot six times a year over a daily routine. Before starting injections, your provider may have you take an oral version of the same drug for four weeks to make sure you tolerate it well.
On-Demand Dosing
For cisgender men who have sex with men, there’s also an event-driven option sometimes called “2-1-1” dosing. Instead of taking a pill daily, you take two pills between 2 and 24 hours before sex (closer to 24 hours is better), then one pill 24 hours after that first dose, and one more pill 24 hours later. If you’re having sex again within seven days of your last dose, you only need one pill before sex instead of two, followed by the same two follow-up pills. This approach uses the same medication as daily oral PrEP but in a flexible schedule. It is not recommended for people at risk through vaginal sex or injection drug use.
How Long Until You’re Protected
PrEP doesn’t work instantly. The drug needs time to build up to protective levels in the tissues where HIV would enter your body, and that timeline varies. For receptive anal sex, studies show the drug reaches maximum concentration in rectal tissue after about 7 days of daily dosing. For vaginal exposure, it takes significantly longer: up to 20 days of consistent daily use before the drug fully saturates cervicovaginal tissue. During this ramp-up period, using condoms or other prevention methods adds an important layer of protection.
Who Can Take PrEP
Current CDC guidance takes a broad approach: PrEP should be offered to any sexually active person who doesn’t have HIV and asks for it, even if they don’t report specific risk factors. This is a notable shift from earlier guidelines that required clinicians to assess specific risk behaviors before prescribing.
Some situations where PrEP is particularly relevant include having a sexual partner whose HIV status is unknown, having a partner with HIV who isn’t consistently virally suppressed, having multiple sexual partners, or sharing injection equipment. If your partner with HIV is on treatment and consistently virally suppressed, PrEP still shouldn’t be withheld if you want the added reassurance.
There are a few situations where certain forms of PrEP aren’t appropriate. People who already have HIV should not take PrEP, because using only one or two antiretroviral drugs when the virus is already present can lead to drug resistance. Oral PrEP isn’t suitable for people with severe kidney impairment. Injectable PrEP shouldn’t be used by anyone who has had an allergic reaction to cabotegravir, its active ingredient.
Side Effects and Monitoring
Most people tolerate PrEP well. The most commonly reported side effects when starting oral PrEP are nausea, headache, and fatigue, and these typically fade within the first few weeks. Serious side effects are uncommon but worth understanding.
The older oral formulation (Truvada and its generics) can affect kidney function and bone mineral density over time. The newer formulation (Descovy) has shown measurably better outcomes on both of these markers in clinical studies, with significant differences in hip and spine bone density and in several kidney biomarkers at 48 weeks. For most healthy people, these effects with either formulation are modest and reversible after stopping the drug, but your provider will check kidney function before you start and periodically while you’re on it.
You’ll also need an HIV test before starting PrEP and at regular follow-up visits. This confirms you remain HIV-negative, because taking PrEP while unknowingly HIV-positive could allow the virus to develop resistance to the drugs.
Cost and Insurance Coverage
In October 2024, federal agencies clarified that under the Affordable Care Act, most employer-sponsored and marketplace health plans must cover PrEP without any cost-sharing. This includes daily oral PrEP (both formulations) and the long-acting injectable. That means no copays, no deductibles, and no coinsurance for the medication itself, along with the associated lab work and clinic visits needed to prescribe and monitor it.
If you’re uninsured, manufacturer assistance programs and the federal Ready, Set, PrEP program can provide the medication at no cost. Generic versions of Truvada have also driven prices down substantially, making oral PrEP more accessible than it was even a few years ago.
PrEP and Other Prevention Methods
PrEP is highly effective on its own, but it only protects against HIV. It doesn’t prevent other sexually transmitted infections like gonorrhea, chlamydia, or syphilis. Condoms remain the only method that reduces risk across all STIs. Many people use PrEP alongside condoms, while others rely on PrEP alone for HIV prevention and get regular STI screening. Your approach depends on your circumstances and what you’re comfortable with. The key factor for effectiveness is consistency: PrEP only works as well as the data suggests when it’s taken on schedule.