PrEP (pre-exposure prophylaxis) is HIV medicine that people without HIV take to stay HIV-negative. Taken as prescribed, it cuts the risk of getting HIV from sex by about 99% and from injection drug use by at least 74% CDC PrEP. It comes as a daily pill or, now, a long-acting shot, and it's one of the CDC's core prevention tools.
flu-like; many have none
when undetectable
| Item | Value |
|---|---|
| Acute symptoms | 2–4 wks — flu-like; many have none |
| NAT detects | 10–33 days |
| Antibody test | 23–90 days |
| U=U | no transmission — when undetectable |
How do you actually prevent HIV?
HIV spreads through only a few body fluids — blood, semen, vaginal fluid, rectal fluid, and breast milk — passed mostly through anal or vaginal sex, shared needles, or from a parent during pregnancy, birth, or breastfeeding CDC. It does not spread through saliva, kissing, food, surfaces, insects, or air, because the virus doesn't survive long outside the body. Block those few routes and you block transmission.
The CDC stacks several tools, and they're stronger together than alone:
- PrEP — daily pills or a long-acting shot for HIV-negative people at ongoing risk.
- PEP — a 28-day emergency course after a possible exposure, started within 72 hours.
- Treatment as prevention (U=U) — a partner with HIV who stays virally suppressed cannot transmit it.
- Condoms — a barrier against HIV and most other STIs.
- Regular testing — so HIV is caught early and prevention is built on a known status.
Who should take PrEP, and how well does it work?
PrEP is for anyone HIV-negative who could be exposed through sex or injection drug use — for example, someone with a partner living with HIV, anyone with multiple partners or inconsistent condom use, and people who share injection equipment. You don't need a 'reason' that fits a stereotype. If HIV is on your radar, it's a reasonable conversation with a clinician.
How well it protects tracks almost entirely with how you take it. In the iPrEx trial, daily pills cut HIV risk about 92% in people who actually took them, even though overall protection was lower because some didn't. On-demand '2-1-1' dosing reached 86% in IPERGAY, and the long-acting cabotegravir shot beat daily pills by 66% in HPTN 083. Twice-yearly injectable lenacapavir produced zero infections among women in the PURPOSE 1 trial WHO. PrEP only works when it's in your system.
The PrEP options
There are two daily pills and one injectable, and they're not interchangeable for everyone.
| Option | How it's taken | Approved for |
|---|---|---|
| Truvada (emtricitabine/tenofovir DF) | One pill daily | All exposure routes — sex (including receptive vaginal) and injection drug use |
| Descovy (emtricitabine/tenofovir AF) | One pill daily | Sex — but NOT receptive vaginal sex or injection drug use (its trials didn't include those groups) |
| Cabotegravir (Apretude) | Injection: two starter doses a month apart, then every 2 months | People at risk through sex |
Descovy isn't approved for people exposed through receptive vaginal sex or for those who inject drugs, because the studies that got it approved didn't enroll cisgender women or people who inject. Truvada covers all of those routes.
What '2-1-1' on-demand dosing means
For men who have sex with men who don't want a daily pill, on-demand PrEP is an option: two pills 2 to 24 hours before sex, one pill 24 hours after the first dose, and one more 24 hours after that. This is the regimen that reached 86% protection in IPERGAY. It's only validated for this group and only with Truvada, and it isn't a substitute for daily dosing in people who'd benefit from receptive-vaginal coverage.
What starting and staying on PrEP is actually like
You can't start PrEP without a confirmed HIV-negative test. Starting PrEP while HIV is quietly already present can breed hiv drug resistance, leaving fewer working drugs later. Baseline visits also check kidney function, hepatitis B, and other STIs. After that, oral PrEP needs a follow-up roughly every 3 months and the injectable every 2 months — an HIV test, refills, and STI screening each time. If a daily pill or quarterly visits feel like a lot, the long-acting injectable is built to ease that ongoing structure, and it overlaps with the science behind injectable hiv treatment for people already living with HIV.
Condoms — what they cover, and where they don't
Condoms are still a workhorse: used correctly, a barrier blocks the fluids that carry HIV and also lowers risk for gonorrhea, chlamydia, and other infections PrEP does nothing against. PrEP protects against HIV only, so condoms fill the gap it leaves. The common failure isn't the latex, it's real life. A condom used late, not at all, or that slips offers no protection. Most clinicians treat condoms as one layer, not the whole plan.
Testing as prevention
Knowing your status — and your partner's — is the foundation everything else sits on. In 2023, about 38,800 people were newly diagnosed with HIV in the US, and roughly 1.12 million are living with it, of whom about two-thirds are virally suppressed CDC AtlasPlus, 2023. People who test, treat, and reach undetectable aren't transmitting.
Timing matters because tests have a window — the gap after exposure before they turn positive. A nucleic-acid test can detect HIV roughly 10 to 33 days out, a 4th-generation antigen/antibody lab test 18 to 45 days, and antibody/rapid tests 23 to 90 days CDC. A negative is only conclusive after the window closes with no exposure during it. If you're unsure of the math, here's the guide on when to test after exposure, and you can get tested without an established diagnosis or symptoms.
U=U, PEP, and the newest prevention science
Undetectable equals untransmittable (U=U)
A person with HIV who takes their medicine and stays virally suppressed (under 200 copies/mL) does not transmit HIV to sex partners. Across the PARTNER, Opposites Attract, and PARTNER2 studies, mixed-status couples logged more than 125,000 condomless sex acts with zero linked transmissions while the partner was undetectable PARTNER study. Most people reach undetectable within about six months of starting treatment CDC U=U. For a serodifferent couple, a treated, suppressed partner is itself a prevention method.
PEP — the 72-hour emergency window
If a possible exposure already happened — a condom broke, a needle was shared, an assault — PEP can still prevent infection, but only if it starts within 72 hours and continues daily for 28 days CDC PEP. In the original occupational study it cut seroconversion by about 81%. This is an urgent-care or ER conversation tonight, not a wait-and-test one. PEP is for emergencies and isn't a stand-in for PrEP or condoms. If you find yourself needing PEP repeatedly, that's the signal to start PrEP.
Where prevention research is heading
Beyond lenacapavir's twice-yearly shot, scientists are studying why a rare genetic trait blocks the virus at all — see resistance to hiv infection - how a mutation may help fight aids, which involves the same CCR5 doorway some experimental cure work targets. These aren't options you can ask for yet, but they're why long-acting prevention keeps getting easier.
Putting it together
The strongest plan layers tools to your own situation rather than betting on one. If you're HIV-negative with ongoing risk, PrEP is the centerpiece, with condoms covering the other STIs it can't. If your partner has HIV, their staying virally suppressed (U=U) plus your PrEP gives belt-and-suspenders protection. If something already happened, PEP within 72 hours is the move. Regular testing keeps the whole plan honest.
When to see a clinician
Reach out promptly if any of these apply:
- You may have been exposed in the last 72 hours — ask about PEP today; the window closes fast.
- You'd benefit from ongoing protection — to start PrEP and get the baseline HIV, kidney, hepatitis B, and STI screening.
- Flu-like symptoms 2 to 4 weeks after a risk — about 90% of people get them in acute HIV, exactly when the virus is most contagious, so get an urgent test rather than waiting it out CDC.
- You're pregnant or planning to be and could be at risk — treatment in pregnancy can cut transmission to a newborn to under 1%.