Short answer
PrEP and PEP both help prevent HIV, but the timing is different. PrEP is planned prevention for people who do not have HIV and may have future exposure. PEP is emergency prevention after a possible exposure and should be evaluated as soon as possible, within 72 hours. Testing also differs: PrEP has recurring HIV and STI monitoring, while PEP starts with urgent baseline testing and follow-up HIV testing after the exposure.
PrEP versus PEP at a glance
| Question | PrEP | PEP |
|---|---|---|
| What is it for? | Ongoing prevention before possible HIV exposure. | Emergency prevention after a possible HIV exposure. |
| Timing | Started before or during a period of ongoing risk. | Evaluated rapidly when care is sought within 72 hours after exposure. |
| Medication timeline | Daily oral medication or scheduled injections, depending on the PrEP option. | CDC describes a 28-day course when PEP is prescribed. |
| First HIV test | HIV status must be confirmed before starting or continuing PrEP. | Baseline HIV testing is part of evaluation, but indicated PEP should not be delayed if rapid testing is unavailable. |
| Follow-up HIV testing | Usually every 3 months for oral PrEP; injectable PrEP has a 1-month visit and then at least every 2 months. | CDC's PEP page summarizes follow-up HIV testing at 30 and 90 days; CDC guidelines describe repeat testing at 4 to 6 weeks and 3 months after exposure. |
| STI testing | Part of ongoing PrEP care and may include urine, throat, rectal, vaginal, or blood tests based on exposure. | Often part of the baseline exposure evaluation and follow-up plan, depending on what happened and symptoms. |
The 72-hour PEP window
CDC describes exposure to HIV as a medical emergency because HIV can establish infection quickly. People should be evaluated rapidly for PEP when care is sought within 72 hours after a potential exposure. PEP is not recommended for exposures that happened more than 72 hours before seeking treatment because it is unlikely to be effective after that time.
If rapid HIV blood testing is not available and PEP is indicated, CDC says the first dose should be given immediately. PEP can be stopped later if testing shows the person already has HIV or if the source is determined not to have HIV. In practical terms, an urgent PEP visit is not the time to wait several days for routine lab results before asking for care.
What tests may happen at a PEP visit
CDC PEP clinical guidance lists baseline screening before starting PEP, including a baseline HIV rapid test, pregnancy testing when relevant, serum liver enzyme testing, and blood urea nitrogen/creatinine testing. Depending on the exposure, clinicians may also consider STI testing, hepatitis B and C testing, vaccination needs, emergency contraception, or care after sexual assault. For that situation, see the STI testing after sexual assault guide.
An oral HIV test is not recommended for PEP evaluation. Blood-based testing is preferred because the decision is time-sensitive and early infection can be harder to detect.
PEP follow-up HIV testing
PEP follow-up is how clinicians check whether HIV infection occurred after the exposure and support completion of the medication course. CDC's PEP page describes follow-up HIV testing at 30 and 90 days. CDC's detailed guidelines describe repeat HIV testing at 4 to 6 weeks and 3 months after exposure. Some people may need additional follow-up if hepatitis C is acquired at the same time because HIV test detection can be delayed in that situation.
PrEP follow-up testing
PrEP is not a one-time exposure response. For oral PrEP, CDC lists repeat HIV antigen/antibody and HIV-1 RNA testing at least every 3 months, along with symptom review, adherence support, and STI testing when indicated. For injectable cabotegravir PrEP, CDC lists HIV antigen/antibody and HIV-1 RNA testing at the 1-month visit after the first injection and then at least every 2 months beginning in month 3.
Medication-specific labs also matter. Oral PrEP can require kidney function checks and hepatitis B screening, and F/TAF PrEP can require cholesterol and triglyceride monitoring. For the full ongoing checklist, see the PrEP labs and STI testing follow-up guide.
Transitioning from PEP to PrEP
If a person has frequent or ongoing HIV exposure risk, CDC says PrEP should be considered. If the most recent possible exposure was within the 72-hour window, PEP may be indicated first, and PrEP may be prescribed after completing the 28-day PEP course. That transition is a clinical plan: it should include HIV testing, symptom review, STI screening, medication choice, kidney or hepatitis context, and a clear schedule for follow-up.
STI testing is still separate
Neither PrEP nor PEP prevents chlamydia, gonorrhea, syphilis, herpes, hepatitis, HPV, trichomoniasis, BV, yeast, or other infections. Depending on the exposure, STI testing may need blood tests, urine tests, vaginal or cervical swabs, throat swabs, or rectal swabs. If oral or anal sex was part of the exposure, ask whether body-site testing is included. A urine-only test can miss throat or rectal infection.
Questions to ask during care
- Based on the exposure timing, should I be evaluated for PEP today?
- Which HIV test is being used now, and when should I repeat HIV testing?
- If I start PEP, what is the plan for finishing the 28-day course and checking labs?
- Should I discuss PrEP after PEP because future exposure is possible?
- Which STI tests are included, and which body sites are being tested?
- Do I need pregnancy testing, hepatitis B or C testing, kidney labs, liver enzymes, vaccines, or emergency contraception?
Related Lab Intel guides
For condom failure or a possible exposure, see the STI testing after condom break or possible exposure guide. For HIV window periods, see the HIV testing window period guide. For sexual assault or nonconsensual exposure, see the STI testing after sexual assault guide. For anal-exposure questions, see the STI testing after anal sex guide. For ongoing PrEP labs, see the PrEP labs and STI testing follow-up guide. For testing after a new partner, see the STI testing after a new partner guide. For planned condom decisions with a partner, see the STI testing before stopping condoms guide. For what a broad panel may miss, see the full STI panel guide. For hepatitis markers, see the hepatitis B and C testing guide. For throat and rectal swabs, see the extragenital STI testing guide. For affordable access, see the free and low-cost STI testing guide.