HIV PEP: The 72-Hour Window Explained

Why does the 72-hour limit exist?

PEP works by interrupting HIV before it can establish a permanent infection. The antiretroviral medications in the regimen prevent the virus from replicating in your body — but only if started early enough. After 72 hours, the virus has had sufficient time to integrate into cells, and PEP is no longer recommended 1Ref 1Workowski KA, Bachmann LH, Chan PA, Johnston CM, Muzny CA, Park I, Reno H, Zenilman JM, Bolan GA (2021).Sexually Transmitted Infections Treatment Guidelines, 2021.The 72-hour PEP window, the 28-day antiretroviral regimen, and baseline HIV testing before initiating PEP.

The window is not a cliff you fall off at exactly 72 hours; effectiveness starts declining from the moment of exposure and drops significantly over time. Starting at hour 4 is meaningfully better than starting at hour 60. If you are at or near the limit, go now rather than gathering more information first.

What PEP actually is — and what it is not

PEP is a 28-day course of antiretroviral medications — the same class used to treat HIV in people who are already positive. You take them every day for the full 28 days without interruption. Missing doses or stopping early reduces effectiveness significantly.

Common side effects include nausea and fatigue, which often ease after the first week. A clinician will walk you through the regimen and monitor you during the course 1Ref 1Workowski KA, Bachmann LH, Chan PA, Johnston CM, Muzny CA, Park I, Reno H, Zenilman JM, Bolan GA (2021).Sexually Transmitted Infections Treatment Guidelines, 2021.The 72-hour PEP window, the 28-day antiretroviral regimen, and baseline HIV testing before initiating PEP.

PEP is an emergency measure, not a routine prevention strategy. It is not designed for repeated use. If you find yourself needing PEP more than once, or if you have ongoing exposure risk, ask about PrEP — a daily medication taken before exposure that substantially reduces HIV risk when taken consistently 2Ref 2Centers for Disease Control and Prevention (2021).Preexposure Prophylaxis for the Prevention of HIV Infection in the United States – 2021 Update: A Clinical Practice Guideline.Recommendation to offer PrEP following PEP completion when ongoing exposure risk is present.

Where to get PEP right now

• Emergency rooms: able to prescribe PEP 24 hours a day, seven days a week — always the right choice outside business hours
• Urgent care centers: most can prescribe it; call ahead if you have time, but do not let a phone call delay you
• Sexual health clinics and HIV clinics: often the most experienced providers; may have same-day appointments
• Primary care providers: can prescribe if they can see you the same day

If you have trouble locating a same-day source, the CDC's HIV helpline (1-800-232-4636) and your local health department can direct you. The important thing is to go — not to find the ideal setting.

What happens when you start PEP

Your clinician will test you for HIV at the start — to confirm you were HIV-negative before the exposure, because treatment is different if you already have the infection 1Ref 1Workowski KA, Bachmann LH, Chan PA, Johnston CM, Muzny CA, Park I, Reno H, Zenilman JM, Bolan GA (2021).Sexually Transmitted Infections Treatment Guidelines, 2021.The 72-hour PEP window, the 28-day antiretroviral regimen, and baseline HIV testing before initiating PEP. They will also typically screen for hepatitis B and C and for other STIs, since the same exposure event may carry additional risk.

Follow-up HIV testing typically happens at the end of the 28-day course, and again at 45 and 90 days after the exposure. This is to confirm that PEP was effective and that you remain HIV-negative.

The visit is also a natural time to discuss PrEP. The CDC's clinical guidance on PrEP recommends that clinicians offer it to anyone who is completing a PEP course and who has ongoing exposure risk 2Ref 2Centers for Disease Control and Prevention (2021).Preexposure Prophylaxis for the Prevention of HIV Infection in the United States – 2021 Update: A Clinical Practice Guideline.Recommendation to offer PrEP following PEP completion when ongoing exposure risk is present.

Does the type of exposure affect whether PEP is recommended?

Yes. Receptive anal intercourse, sharing injection needles, and needle-stick injuries involving HIV-positive blood carry higher transmission risk than other exposures. Receptive oral sex carries lower risk. A clinician will consider the exposure type and, when possible, the HIV status of the source person when counseling you.

If the source person is HIV-positive but on treatment with a confirmed undetectable viral load, transmission risk is very low — though clinicians typically still discuss PEP given the stakes. When the source person's status is unknown, treat it conservatively and seek care promptly.

The right approach is not to calculate your own risk level and decide whether you 'qualify.' That calculation belongs with a clinician who can ask the right questions.