PEP in 72 Hours — A Decision Tree After Possible HIV Exposure

Post-exposure prophylaxis (PEP) is a 28-day course of antiretroviral drugs that can prevent HIV from establishing infection after a possible exposure — but only if you start it fast. The clock is 72 hours from the exposure event, and every hour inside that window matters. This guide walks the decision tree: whether you actually need PEP, where to get it in the country you're standing in right now, what it costs, what it feels like, and what testing looks like after the 28 days are done.

The 72-Hour Window: Why Timing Is Everything

HIV needs roughly 24 to 72 hours after entering the body to migrate from the initial site of exposure to regional lymph nodes and establish systemic infection. PEP works by loading your system with antiretrovirals during that window so that any virus that has entered cannot replicate. Once systemic infection is established, PEP no longer helps.

0–2 hours: Optimal. Efficacy is highest.
2–24 hours: Still strongly recommended by every major guideline (CDC, WHO, BHIVA, EACS).
24–72 hours: Recommended but with diminishing returns as the hours pass.
After 72 hours: Not recommended. There is no evidence that starting PEP past this point prevents seroconversion.

Practical implication: if you think you may have been exposed, go to an emergency department or sexual health clinic tonight, not tomorrow morning. Do not wait to "see how you feel." Do not wait for the source person to get tested — you can start PEP and stop it later if the source turns out to be HIV-negative. Do not wait for a clinic appointment if a walk-in ER can see you sooner.

Risk Assessment: Does This Exposure Actually Warrant PEP?

PEP is not a morning-after pill for every sexual encounter. Clinicians weigh three variables: the HIV status of the source, the type of exposure, and any modifying factors like viral load or condom failure.

Source status matters most. If the source person is known HIV-positive with an undetectable viral load (on effective treatment for at least six months), the transmission risk is effectively zero — this is the U=U principle (Undetectable = Untransmittable), backed by the PARTNER and Opposites Attract studies. PEP is generally not indicated in this scenario. If the source is HIV-positive with unknown or detectable viral load, PEP is strongly indicated for any mucosal or needle exposure.

If source status is unknown — the most common real-world scenario — clinicians assess based on the source's likely risk profile and the type of exposure:

PEP generally recommended: Receptive anal sex without a condom (or condom failure), shared injection equipment, needlestick from a hollow-bore needle with visible blood, sexual assault.
PEP considered case-by-case: Insertive anal sex, receptive vaginal sex, oral sex with ejaculation and open sores or recent dental work, exposure in a high-prevalence setting (sub-Saharan Africa, some populations in Southeast Asia and Latin America).
PEP generally not recommended: Oral sex without ejaculation, mutual masturbation, exposure to intact skin, kissing, condom-protected sex where the condom did not fail.

Two modifiers push toward starting PEP: presence of another STI (ulcerative STIs like syphilis and herpes raise transmission risk substantially) and menstruation or genital trauma at the time of exposure. When in doubt, start. You can always stop the course after 24–48 hours if further information rules out risk.

Where to Get PEP, By Setting

Access varies dramatically by country. General patterns:

United States: Any hospital emergency department is required to evaluate for PEP. Sexual health clinics, Planned Parenthood, and many urgent care centers also prescribe. Some pharmacies in California, Colorado, New York, and a handful of other states can dispense PEP directly under standing orders — check locally.
United Kingdom: Free on the NHS. Sexual health (GUM) clinics during opening hours; A&E (emergency departments) after hours. Some clinics also offer PEPSE (PEP after sexual exposure) via same-day appointments.
European Union: Widely available at hospital ERs and dedicated HIV/STI clinics. Generally free or heavily subsidized for residents; travelers may pay out of pocket in some countries.
Australia and New Zealand: Free through sexual health clinics and hospital emergency departments for citizens and permanent residents; travelers pay but costs are moderate.
Thailand: Available at major private hospitals in Bangkok, Chiang Mai, and Phuket, and at the Thai Red Cross Anonymous Clinic in Bangkok, which is a well-regarded resource for travelers. Costs are far lower than the US.
Rest of Southeast Asia, Latin America, Africa: Availability is uneven. Major cities usually have access at large private hospitals or NGO clinics; rural areas often do not. Travelers should identify the nearest PEP-dispensing facility before high-risk activities, not after.

If you are traveling, the international AIDS Society and country-specific HIV NGOs maintain updated clinic lists. Save one before you leave.

What PEP Actually Is: The Regimen

Modern PEP is a fixed 28-day course, typically a two- or three-drug combination taken once daily:

Backbone: Tenofovir disoproxil fumarate (TDF) 300 mg plus emtricitabine (FTC) 200 mg, sold as a single combined pill (brand name Truvada, or generic equivalents in most countries).
Third agent: Usually dolutegravir 50 mg (Tivicay) or raltegravir 400 mg twice daily (Isentress). Older regimens used efavirenz, which had significant CNS side effects and is no longer preferred.

Total pill burden is typically two pills once a day (or three if raltegravir is used). The 28-day course is not negotiable — stopping early risks incomplete suppression and, if HIV was in fact transmitted, potential resistance in the virus that establishes.

Cost and Access Without Insurance

Sticker prices are frightening; actual costs paid are usually much lower if you know where to look.

United States without insurance: List price for a 28-day course runs roughly $600 to $1,200 for the medication alone, plus ER or clinic fees. However, Gilead's Advancing Access program provides free PEP medication to uninsured people who qualify — application takes minutes and clinics can initiate it. Many state and city health departments also cover PEP at no cost through 340B pricing.
United States with insurance: Usually covered with standard copays. Ryan White program picks up gaps for lower-income patients.
UK, most of EU, Australia, Canada: Free or nominal fee for residents; travelers pay but usually a fraction of US prices.
Generic-market countries (Thailand, India, South Africa, Brazil, Mexico): A 28-day course of generic TDF/FTC plus dolutegravir often costs under $100, sometimes far less.

If you are quoted a five-figure ER bill for PEP in the US, that is a billing artifact, not what the drugs actually cost. Ask the social worker or pharmacist about assistance programs before you leave the building.

Side Effects and Adherence

Modern PEP is dramatically better tolerated than the AZT-based regimens of the 1990s. Most people finish the 28 days without stopping. Expect:

First week: Nausea, fatigue, headache, and occasionally loose stools are common. Taking pills with food helps. Anti-nausea medication (ondansetron) is worth asking for up front.
Ongoing: Tenofovir can affect kidney function and bone density; baseline creatinine testing is standard. Dolutegravir occasionally causes insomnia or vivid dreams; taking it in the morning instead of evening usually resolves this.
Rare but serious: Hypersensitivity reactions, hepatitis flares in people with active hepatitis B (stopping tenofovir suddenly can cause a rebound), significant kidney dysfunction. These require medical attention, not stoicism.

Missing doses is the main way PEP fails. Set two daily alarms. If you miss a dose by less than 24 hours, take it as soon as you remember. If it has been longer, take the next scheduled dose and do not double up. If you vomit within an hour of taking a pill, take another.

Follow-Up Testing: The Schedule That Actually Matters

PEP is not a one-visit intervention. Testing schedule (per CDC and BHIVA guidelines):

Baseline (day 0): Fourth-generation HIV antigen/antibody test, hepatitis B and C serology, syphilis, gonorrhea and chlamydia at exposed sites, pregnancy test where relevant, creatinine and liver function.
Week 4–6 (after finishing PEP): Repeat fourth-gen HIV test. This is the key follow-up.
Week 12 (three months post-exposure): Final HIV test. A negative result here effectively rules out seroconversion from the original exposure with modern fourth-generation assays.
Some guidelines still recommend a 6-month test for people also exposed to hepatitis C, or if there is ongoing risk.

Between the exposure and the three-month clear test, use condoms with any partners and do not donate blood. If any acute retroviral syndrome symptoms develop — fever, rash, sore throat, lymph node swelling, roughly 2–4 weeks post-exposure — get tested immediately rather than waiting for the scheduled visit.

After PEP: The Transition to PrEP

If you needed PEP once, you may benefit from pre-exposure prophylaxis (PrEP) — a daily or on-demand pill taken before potential exposures rather than after. PrEP is roughly 99% effective at preventing HIV from sex when taken as prescribed, is much cheaper than repeated PEP courses, and eliminates the 72-hour panic.

Standard transitions:

Finish the 28-day PEP course.
Confirm HIV-negative status at the week 4–6 test.
Roll directly into daily oral PrEP (TDF/FTC or TAF/FTC) or, for men who have sex with men, on-demand "2-1-1" dosing (two pills 2–24 hours before sex, one pill 24 hours later, one pill 48 hours after the initial dose).
Injectable PrEP (cabotegravir, brand Apretude) is available in the US, UK, and a growing number of countries — one injection every two months, no daily pill.

PEP exists because prevention sometimes fails, condoms break, and life happens. Using it once is a health decision, not a moral verdict. Using it repeatedly is a signal that PrEP would serve you better. Either way, the operative fact is the 72-hour clock: if you think you may have been exposed, the next appointment you make should be tonight.