PrEP is highly effective at preventing HIV when taken as prescribed. Taken consistently, it cuts the risk of getting HIV from sex by about 99% CDC, Talk PrEP Together and from injection drug use by at least 74%. Effectiveness depends almost entirely on taking it on schedule — missed doses mean less protection.
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. It's medicine that HIV-negative people take before a possible exposure to keep the virus from taking hold in the body. The key word is before — PrEP is prevention, not treatment, and it's meant to be in your system ahead of any contact with HIV.
Here's the mechanism: HIV needs to enter your cells and start copying itself to establish an infection. The drugs in PrEP block the enzyme the virus uses to replicate, so even if HIV gets into your body, it can't gain a foothold. That protection only exists if there's enough medicine already circulating, which is why timing and consistency matter so much.
Don't confuse PrEP with PEP. PEP (post-exposure prophylaxis) is the emergency course you start after a single possible exposure, ideally within hours; PrEP is the ongoing option you take when exposure is a regular part of your life. They aren't interchangeable — if you've already had a risky exposure and aren't on PrEP, PEP is the right call. You can read more about that timing under when to test after exposure.
How well does PrEP actually work?
The headline numbers come with one condition attached: taken as prescribed. Effectiveness varies by how you might be exposed and by how reliably you take it.
- From sex: PrEP reduces the risk of getting HIV by about 99% when taken as prescribed.
- From injection drug use: PrEP pills reduce the risk by at least 74% when taken as prescribed.
- With inconsistent dosing: protection drops sharply — the drug levels in your body fall, and so does your defense. Adherence is the single biggest factor in whether PrEP works for you.
PrEP also doesn't work instantly. It takes time to build up to full protection in different tissues, and that timeline depends on the route of exposure.
| Route of exposure | Time to maximum protection |
|---|---|
| Receptive anal sex | About 7 days |
| Receptive vaginal sex | About 21 days |
| Injection drug use | About 21 days |
That ramp-up window is exactly why PrEP can't be used as last-minute or morning-after protection — there isn't enough medicine on board yet. If you start today, plan on using condoms or other protection until you reach the timeline above for your situation.
How to use PrEP and who it's for
PrEP comes as a daily pill or a long-acting shot. The right choice depends on your risk and your preferences.
- Truvada — a daily oral pill for people at risk through sex or through injection drug use.
- Descovy — a daily oral pill for people at risk through sex only; it's not recommended for people assigned female at birth who are at risk through receptive vaginal sex.
- Apretude (cabotegravir) — an injectable for people at risk through sex who weigh at least 77 pounds (35 kg). It's a good fit if you'd rather not take a daily pill.
Some people take pills on a non-daily schedule around sex. That approach, sometimes called the 2-1-1 method, is covered in detail under on-demand prep — it isn't right for everyone, so talk it through with a clinician.
In practice, starting PrEP means an HIV test first to confirm you're negative, followed by regular check-ins while you stay on it. Those visits track your kidney function, screen for other STIs, and confirm you're still HIV-negative. PrEP is prescribed at clinics and through telehealth services, so you don't always need an in-person appointment to begin.
Cost and how to get it
PrEP is widely accessible, and cost is rarely the dealbreaker people fear. Many insurance plans cover it, and there are assistance programs designed to cover the cost for people who are uninsured or underinsured. A clinician or PrEP navigator can connect you to those programs when they write your prescription.
Telehealth has made starting easier than it used to be: you can do the initial HIV test, the visit, and ongoing monitoring largely from home in many states, with lab work done locally.
What PrEP does NOT protect against
This is where people get caught off guard. PrEP is built for HIV and HIV only. It does not protect against other STIs like chlamydia, gonorrhea, or syphilis, and it does not prevent pregnancy.
A common and costly mistake is treating PrEP as a do-everything shield. Because it doesn't cover other infections, people on PrEP still need to test regularly — many do so at the same check-ins where they pick up refills. Routine screening is part of staying on PrEP, not an optional add-on. If you're due, you can get tested for the infections PrEP leaves uncovered.
How PrEP fits with other prevention
PrEP works best as one layer in a stack, not a standalone fix. Condoms still matter — they're the only method here that also reduces other STIs and prevents pregnancy. Regular testing catches anything PrEP doesn't cover early, when it's easiest to treat.
There's also a community angle: when people living with HIV are on effective treatment and reach an undetectable viral load, they don't transmit the virus sexually. So your partners' care matters too — earlier hiv treatment can help prevention explains how treatment and prevention reinforce each other.
When to talk to a clinician
Bring up PrEP if you have an HIV-positive partner, multiple partners, inconsistent condom use, a recent STI, or if you share injection equipment. You don't need to justify your risk in detail — clinicians are used to having this conversation, and it starts with a simple HIV test.
If you think you've already had a possible exposure and aren't on PrEP, don't wait to start it — that's a PEP situation, and PEP works only if started quickly CDC, Preventing HIV with PEP. After PEP, you can transition to PrEP for ongoing protection.