Most people on PrEP feel fine. The classic side effects are mild and short-lived: nausea, headache, stomach upset, or fatigue in the first few weeks as your body adjusts, and these usually fade on their own. PrEP also has lab-monitored effects on the kidneys and bone that your clinician tracks with routine bloodwork. Serious problems are uncommon.
risk reduction, taken as prescribed
daily Truvada/Descovy or the Apretude injection
not other STIs or pregnancy
| Item | Value |
|---|---|
| From sex | ~99% — risk reduction, taken as prescribed |
| From injection use | ≥74% |
| Forms | pill or shot — daily Truvada/Descovy or the Apretude injection |
| Protects against | HIV only — not other STIs or pregnancy |
What PrEP is and how it works
PrEP stands for pre-exposure prophylaxis — medicine taken by HIV-negative people before a possible exposure to keep the virus from establishing an infection CDC, Talk PrEP Together. It prevents infection rather than treating it. The drug keeps a protective level of medication in your blood and tissues so that if HIV does enter your body, it can't take hold and replicate.
Timing and consistency matter for the same reason. PrEP only works when there's enough drug on board at the moment of exposure, so it's taken on an ongoing schedule rather than as a one-off. If you've already had a possible exposure and want emergency protection, that's a different medicine — PEP, taken after the fact CDC, Preventing HIV with PEP. PrEP and PEP are not interchangeable: one is the daily seatbelt, the other is the ambulance.
Side effects: what to expect and when
PrEP side effects fall into two buckets: the short-lived "start-up" symptoms in the first weeks, and the slow, lab-tracked effects that matter over months and years.
Start-up symptoms (the first few weeks)
When people first start oral PrEP, some notice nausea, a queasy stomach, headache, tiredness, or loose stools. This is sometimes called a "start-up syndrome" as your gut and your system adjust to a new daily medicine. For most people it's mild, doesn't stop them from going about their day, and settles within the first weeks without any special treatment. Taking the pill with food often eases the nausea. If symptoms are severe or hang on past the first month, call your clinician rather than quietly stopping the medicine.
The slower, lab-monitored effects
Oral PrEP can have small effects on the kidneys and on bone density over time. These usually cause no symptoms at all, so your clinician checks kidney function with routine bloodwork while you're on PrEP. For most people these changes are minor and reverse if PrEP is stopped. The injectable option, cabotegravir, works differently and doesn't carry the same kidney and bone considerations; its most common complaint is a sore spot or lump at the injection site.
What's actually being checked, and how often
Starting PrEP comes with check-ins. Before you begin, you get an HIV test to confirm you're negative, because PrEP alone isn't enough medicine to treat an existing infection and using it that way can breed resistance. After that, you return periodically for repeat HIV testing, kidney monitoring, and STI screening. Telehealth services and clinics both handle this, and the visits are short.
One common mistake is treating PrEP like a morning-after pill. It only protects when taken on the ongoing schedule, and it doesn't cover other STIs, so people on PrEP still need to get tested regularly, not just at the start.
How well it works (the numbers)
PrEP is one of the most effective HIV-prevention tools available. Taken as prescribed, it reduces the risk of getting HIV from sex by about 99%. For people who inject drugs, oral PrEP lowers the risk by at least 74% when taken as prescribed. Those numbers depend on consistency, and missed doses lower the protection.
Protection also isn't instant. The drug needs time to build up in the right tissues. It takes about 7 days of daily dosing to reach maximum protection for receptive anal sex, and about 21 days for receptive vaginal sex and for injection drug use. Until you hit those marks, use condoms as a backstop.
How to use it and who it's for
PrEP is for HIV-negative people who could be exposed to HIV — through sex with a partner whose status is unknown or positive, through a partner not on effective treatment, or through sharing injection equipment. There are three approved options, and they suit different people.
| Option | Form | Who it's for |
|---|---|---|
| Truvada | Daily oral pill | People at risk through sex or injection drug use |
| Descovy | Daily oral pill | People at risk through sex only — not for people assigned female at birth who are at risk through receptive vaginal sex |
| Apretude (cabotegravir) | Injection | People at risk through sex who weigh at least 77 pounds (35 kg); an option for those who'd rather not take a daily pill |
If you're weighing a daily pill against the shot, our full breakdown of prep for hiv prevention walks through the trade-offs in dosing, monitoring, and lifestyle fit.
Cost and how to get it
You can get PrEP from a primary-care clinician, a sexual-health clinic, or a telehealth service — many will do the initial HIV test and labs remotely or refer you locally. Cost doesn't have to be a barrier. Assistance programs exist to cover the medication and the associated visits for those who qualify, and many insurance plans cover preventive PrEP. Ask the prescriber or clinic up front about which program fits your situation.
What PrEP does NOT protect against
PrEP guards against HIV and only HIV. It does nothing against other sexually transmitted infections like chlamydia, gonorrhea, or syphilis, and it offers no protection against pregnancy. Because rates of other STIs can rise when condoms are dropped, regular STI screening is part of responsible PrEP use.
How PrEP fits with other prevention
PrEP works best as one layer in a stack. Condoms still add protection against the STIs PrEP can't touch. Routine STI testing catches infections early, when they're easiest to treat. Vaccines (such as for HPV and hepatitis B) cover ground PrEP doesn't. And on the partner side, the science is clear that earlier hiv treatment can help prevention — a partner on effective HIV treatment with an undetectable viral load can't pass the virus on through sex.
If you've had a possible exposure and aren't sure whether you're past the start-up window or whether testing will be accurate yet, read up on when to test after exposure before you assume a negative result is final.
When to talk to a clinician
Reach out if start-up symptoms are severe, if nausea or fatigue lasts beyond the first month, or if you have new symptoms you can't explain. Call promptly for signs that need quick attention, like a flu-like illness shortly after a possible exposure, which can be early HIV, or symptoms of kidney trouble like swelling or big changes in how much you urinate. And always loop in your clinician before stopping PrEP, especially if you're still at ongoing risk.