Unprotected Sex and HIV Exposure: PEP Within 72 Hours, Testing and STI Prevention

Condomless sex is not the same as a positive test

An exposure without a condom is a reason for assessment. It is not an HIV result. A partner’s status comes from test information that is actually known, not from a job, an appearance, or sex work. This page does not judge anyone on that basis.

The CDC 2025 guidance recommends PEP when the exposure carries substantial risk and the source has HIV without sustained viral suppression, or that information is unknown. When the source’s HIV status is unknown, the clinician decides case by case and does not hold the first dose in order to investigate the source first.

Risk is not the same for every exposure. The clinician looks at the type of contact, the body fluid, whether a barrier was present, and whether the person is already taking PrEP. This page does not say that every condomless exposure receives the same medicine.

PEP is medicine after the event, not a product to buy alone

PEP, or post-exposure prophylaxis, is antiretroviral medicine used after a possible HIV exposure to reduce the chance that the virus establishes infection. When a clinician judges that it is indicated, the course is 28 days. Missing doses means the protection is no longer the course the guidance studied.

The regimen is chosen from other illnesses, current medicines, kidney and liver results, and source information when it exists. This page does not print a regimen or a dose for self-purchase. If a rapid test already shows HIV, PEP is not started as prevention; that person needs HIV care. If a dose was started because the laboratory result was not back, and HIV is confirmed later, do not stop it on your own.

None of the guidance opened for this page lists food, a vitamin, a herb, or washing the genitals as a substitute for PEP.

Get a baseline HIV test, and do not let that test delay the medicine

A person assessed for PEP should have a baseline HIV test, as the guidance describes: a rapid test, a laboratory antigen/antibody test, or both. The aim is to learn whether HIV was already present, without delaying the first dose.

If a laboratory result is not ready and PEP is indicated, start PEP immediately. Do not wait until the 72-hour window has closed. The medicine can be stopped later if the person is found to have HIV already, or if the source is found not to have HIV. An oral-fluid rapid test is not the right screen for this visit, because it is less sensitive for recent infection than a blood test.

Steps after a possible HIV exposure, and the follow-up tests

• If it has been 72 hours or less, go to an emergency department or a hospital that provides HIV care today.
• Tell the clinician the time, the type of exposure, and current medicines, including PrEP if it is in use.
• Have a baseline HIV test. If the laboratory result is not back and PEP is indicated, the first dose can start.
• If PEP is prescribed, complete 28 days and keep the follow-up visits. A negative test on day one is not a reason to stop.

Where the 4–6 week and 12 week visits start

The CDC 2025 recommendation lines schedule an interim test about 4–6 weeks after the exposure and a final test 12 weeks after the exposure. Both use a laboratory antigen/antibody test and a diagnostic nucleic acid test.

The explanation in the same guidance calls these the visits at about 4–6 weeks and 12 weeks after PEP starts, and it says the final test is about 8 weeks after the 28-day course is finished. These are one plan. PEP has to start within 72 hours of the exposure, so the two starting points are no more than that far apart.

The reading that does not match the guidance is to finish 28 days and then wait another 12 weeks for the last test. A negative result at the 4–6 week visit does not rule out infection, because the medicine can suppress the virus for longer than 2 weeks after it is stopped. A negative result during follow-up is not the final assessment.

The test method follows the guidance and what the hospital can do. Lack of a nucleic acid test is not a reason to withhold PEP; the most sensitive available test is used. The 4–6 week test after PEP starts may be deferred only when PEP began within 24 hours, no doses were missed, and PrEP is not about to start. That is a clinician’s decision, not a reason to skip the visit alone. If the 12-week test is missed, test as soon as possible at the next visit.

How soon ordinary HIV tests can detect infection

CDC describes general window periods separately from the PEP schedule. A laboratory antigen/antibody test on blood from a vein can usually detect HIV 18–45 days after exposure. The same type of test from a finger stick can usually detect it 18–90 days after exposure. A nucleic acid test can usually detect it 10–33 days after exposure. An antibody test can usually detect it 23–90 days after exposure. Those figures are not a replacement schedule for a person taking PEP, because antiretroviral medicine can delay detection.

Hepatitis B and other sexually transmitted infections

Clinicians test for hepatitis B at the start to see whether infection is already present, whether there is immunity or a vaccine history, and whether the regimen is safe. Some antiretroviral medicines used for PEP are also active against hepatitis B. Stopping them can affect a person who already has HBV and can be followed by a hepatitis flare in that group. A flare is not something that happens to every person who stops PEP.

Hepatitis B vaccine, or hepatitis B prevention after an exposure, depends on immune history, the source’s status when it is known, and the relevant guidance. It is not one automatic injection for every exposure.

Syphilis, gonorrhea, and chlamydia, and hepatitis C, are considered according to the exposure. They are not one compulsory panel for every person. Creatinine and liver-enzyme tests help with regimen choice, but the first PEP dose is not held for those results. A pregnancy test is done when the person being assessed can become pregnant.

PEP and PrEP are not the same medicine

PEP is used after an exposure that just happened. PrEP is used before a future exposure, for a person who may be exposed again. Thailand’s 2025 national PrEP guideline, from the Division of AIDS and STIs, says that if the latest exposure was within 72 hours, consider a full 28-day PEP course first, then start PrEP after a negative HIV test, with no gap.

If more than 72 hours have passed and there are no symptoms suggesting acute HIV, that guideline allows PrEP to be considered, with HIV testing emphasized at one month. CDC says the evidence is not sufficient to recommend starting nPEP after 72 hours, although some experts have discussed a longer window. A person who arrives later should still be tested, offered PrEP information, and given a follow-up plan.

A person who is already taking PrEP as prescribed generally does not need PEP added after a sexual exposure. Exceptions include having just started PrEP, missed doses, or exposure to virus with resistance to the PrEP medicines. A clinician makes that call. This page does not teach a pill-counting method for PrEP.

After 72 hours there is still a next step

Get an HIV test, mention fever, rash, or a flu-like illness if those appeared, and ask about tests for other sexually transmitted infections. If a later test confirms HIV, long-term care is a different page. Read living with HIV after the result is known only if that point is reached.

What belongs in the visit, and what does not replace it

Why the start time matters

Asst. Prof. Dr. Norawit Raatpiboon explains that after the virus enters the body it has to enter cells and make copies before infection is established in a way that is hard to reverse. PEP reduces that copying if it starts in time. The evidence used by the guidance shows that protection falls as the start is delayed, which is why 72 hours is the outer limit and why the first hours, especially the first 24, matter most.

A test on the first day cannot see an infection from the night before, because antigen, antibody, or viral nucleic acid is not yet in the range the test detects. If antiretroviral medicine is taken, it can suppress those signals for a further period. The 12-week visit is the guidance’s final assessment, not an extra wait after the pills are finished.

Common questions

Medical limits

This page is general education. It did not examine the person who asked, and it does not replace the clinician who assesses PEP. Do not buy a medicine or change a regimen from this page. If a possible HIV exposure was within the past 72 hours, go to a hospital today.