If a possible HIV exposure happened within the last 72 hours, seek urgent medical care now and ask about PEP.
Post-exposure prophylaxis is a 28-day course of antiretroviral medicine used to reduce the chance of HIV becoming established after a significant exposure. It works best when the first dose is taken as soon as possible—ideally within 24 hours—and should not be delayed while waiting for routine laboratory results.
What to do now
- Write down when the potential exposure occurred.
- Contact a sexual health clinic, emergency department or another service that can provide PEP.
- Tell the clinician what happened, when it happened and whether you were taking PrEP.
- If PEP is recommended, take the first dose without waiting for routine test results. An immediate home test cannot rule out infection from this exposure.
- Complete the prescribed 28-day course and attend the follow-up appointments.
What is PEP?
PEP means post-exposure prophylaxis. “Post-exposure” means after a possible contact with HIV, while “prophylaxis” means prevention. The medicines are antiretrovirals related to those used in modern HIV treatment, but PEP is given for a limited period to someone who does not have HIV.
The aim is to stop the small amount of virus involved in an exposure from spreading through the body and forming a lasting infection. HIV begins establishing itself quickly, so PEP is treated as an emergency intervention rather than something to arrange at a routine appointment several days later.
How quickly must PEP be started?
The 2025 CDC recommendations for nonoccupational HIV PEP advise providing the first dose as soon as possible, ideally within 24 hours and no later than 72 hours after exposure. The recommended course lasts 28 days. A rapid or laboratory HIV test should be performed at the start, but treatment should not be held back while waiting for laboratory results when PEP is indicated.
If more than 72 hours have passed, PEP is generally not recommended because evidence is insufficient to show that starting later is effective. Medical advice still matters: a clinician can assess the exposure, arrange HIV and other infection tests, discuss future prevention and plan follow-up.
Which exposures may need assessment?
PEP assessment may be appropriate when blood, semen, vaginal fluid, rectal fluid or another potentially infectious body fluid has contacted a mucous membrane, broken skin or the bloodstream. Examples include condomless vaginal or anal sex, a condom breaking, sharing injecting equipment, sexual assault or a needlestick injury.
How clinicians assess risk
Risk cannot be determined by a single factor. The clinician will consider:
- the body fluid involved;
- where and how it entered the body;
- whether a barrier such as a condom was used;
- whether the source has HIV and, if so, whether their viral load is durably suppressed;
- whether the exposed person was already taking PrEP correctly;
- the time since exposure.
PEP is not normally recommended after contact with saliva, sweat, tears or urine that is not visibly contaminated with blood. HIV is also not transmitted through everyday contact, hugging, sharing toilets or sharing crockery. When the facts are uncertain, asking for urgent professional assessment is safer than trying to calculate the risk alone.
What if the other person’s HIV status is unknown?
A clinician makes a case-by-case decision using the details of the exposure and what is known about the source. PEP should not be delayed simply to investigate the other person’s status. If the source later tests negative, the prescribing service may advise stopping PEP. The same may apply after a sexual exposure if the source is confirmed to have HIV treatment records showing sustained viral suppression.
No one should pressure another person to disclose medical information or take an HIV test. The exposed person can still be assessed using the information that is available.
What medicines are used?
Current guidelines recommend a combination of antiretroviral medicines for 28 days. Exact regimens differ between countries and are chosen according to age, pregnancy, kidney and liver health, other medicines, possible drug resistance and local availability. The 2025 CDC guideline lists newer preferred three-drug regimens, while the WHO’s 2024 PEP guideline provides a public-health approach for different health systems.
Do not borrow someone else’s HIV medicine or assemble a course from leftover tablets. An assessment checks that the combination and dose are suitable and that there is enough medicine to finish the course.
What happens at the first appointment?
The first appointment usually begins with a focused account of the exposure and a baseline HIV test. The service may also arrange kidney and liver tests, hepatitis B and C testing, and screening for sexually transmitted infections. A pregnancy test may be offered when relevant. These checks help tailor the regimen, but most results do not need to be available before the first dose.
A test performed immediately after an exposure cannot rule out infection from that event. It provides a starting point and may detect an earlier infection. Follow-up testing is needed because laboratory markers of HIV take time to become detectable.
Does PEP always work?
PEP substantially reduces risk, but no clinician can guarantee that it will prevent every infection. There has been no randomised placebo-controlled trial of modern PEP efficacy after sexual exposure; withholding treatment after a significant exposure would be ethically difficult. Recommendations instead draw on animal studies, occupational exposure data, observational PEP programmes, prevention of mother-to-child transmission, and what is known about HIV treatment and PrEP.
The evidence points in the same practical direction: start promptly, use an effective combination and complete the course. Reported infections have often been associated with starting late, not taking the medicine consistently or having further exposures after the original event.
Side effects and missed doses
Modern PEP is generally well tolerated. Nausea, tiredness, headache, diarrhoea and stomach discomfort can occur, particularly near the beginning. Most side effects are manageable and do not last for the whole course. A clinician or pharmacist can suggest ways to reduce them and check for interactions with other medicines.
If a dose is missed, ask the prescribing service or pharmacist what to do. Do not double a dose unless specifically told to do so.
More serious symptoms, an allergic reaction or difficulty keeping tablets down require prompt medical advice. Stopping PEP without speaking to the service may reduce its chance of working.
Testing and precautions after PEP
Follow-up is needed even if every dose was taken. Testing schedules vary, but the CDC’s 2025 recommendations include clinical follow-up and laboratory testing around four to six weeks and 12 weeks after exposure. The local service may adjust timing according to the tests used, hepatitis C risk, pregnancy, further exposures or use of long-acting PrEP.
Until follow-up is complete, the service may advise using condoms, avoiding shared injecting equipment and not donating blood, tissue, organs or semen. PEP does not prevent other sexually transmitted infections or pregnancy, so additional testing, emergency contraception, hepatitis B vaccination or other care may be offered.
PEP after sexual assault
After sexual assault, HIV prevention is only one part of urgent, trauma-informed care. A specialist service can consider PEP without delaying other needs, including treatment for injuries, emergency contraception, testing and prevention for other infections, forensic options and emotional support. The person seeking care remains in control of what happens wherever possible.
From PEP to PrEP
PEP is designed for an unexpected event, not repeated use as the main prevention strategy. Someone who may have continuing or recurring exposure can discuss moving directly to PrEP at the end of the 28-day course. The prescribing service will plan the HIV testing and transition so there is no unnecessary gap.
The difference between PEP and PrEP
The distinction is simple: PEP is started after an urgent exposure, while PrEP is arranged before future exposure. Both require an HIV-negative assessment and appropriate follow-up.
Frequently asked questions
Should I wait for the other person to test?
No. If the exposure may be significant and happened within 72 hours, seek assessment immediately. PEP can be stopped later if reliable information shows it is not needed.
Can a home HIV test tell me whether I need PEP?
No. A home test taken immediately cannot exclude HIV from the recent event. PEP decisions are based on the exposure, timing and clinical assessment as well as baseline testing.
Do I need PEP if I take PrEP?
People who have taken PrEP exactly as prescribed usually do not need PEP. If doses or injections were missed, protection had not yet developed, or the regimen was stopped, urgent assessment is appropriate.
Where can I obtain PEP?
Access differs by country. In the UK, the NHS advises attending a sexual health clinic or A&E after a possible exposure within 72 hours. Elsewhere, emergency departments, HIV clinics and sexual health services are common access points.
The practical conclusion
After possible HIV exposure, the most useful fact is also the simplest: time matters. Seek urgent medical care now if the event was within 72 hours, and go as soon as possible rather than waiting until the deadline. A clinician can decide whether the exposure warrants PEP, begin a suitable 28-day course, arrange testing and help plan future prevention.
FMJ’s Medical Guides summarise journal evidence and recognised clinical guidance for general education. This article cannot calculate an individual’s HIV risk or replace urgent assessment. Please read our medical disclaimer, or report a correction or editorial concern.