What to Do After Unprotected Sex to Prevent HIV

Starting a course of HIV post-exposure prophylaxis, known as PEP, is the single most effective step you can take after unprotected sex to prevent HIV infection. PEP is a 28-day course of antiretroviral medications that can block the virus from establishing itself in your body, but it works best when started within hours of exposure and loses effectiveness the longer you wait. The window, the drugs, the side effects, and the follow-up testing all matter, and getting any of them wrong can undermine the protection PEP offers.

Get to PEP as Fast as You Can

The clock starts the moment exposure happens. PEP must be started within 72 hours to have any chance of working, and every hour of delay chips away at its effectiveness. Modeling research has shown that a standard three-drug PEP regimen can still achieve over 90 percent efficacy when started within 48 hours, but a two-drug backbone alone may not provide adequate protection if initiation is delayed even beyond the first hour or so after exposure. Adding a potent third drug extends the effective window, though early initiation remains the single biggest factor in whether PEP succeeds.¹

This is why the standard advice from clinicians is not “try to start within three days” but rather “go now.” If the exposure happened on a Friday night, do not wait until Monday to see your regular doctor. Emergency departments, sexual health clinics, and urgent care centers can prescribe PEP. Some cities have PEP hotlines or after-hours clinics specifically because the time pressure is so tight. The ideal scenario is walking into a clinic within the first few hours, getting a baseline HIV test and basic blood work, and leaving with a prescription that same visit.

How Risky Was the Exposure

Not every unprotected sexual encounter carries the same risk of HIV transmission, and understanding where your exposure falls on the spectrum can help frame how urgently you need PEP, though it should not be a reason to skip it if you are concerned. A systematic review of per-act transmission risks found that receptive anal intercourse carries the highest sexual transmission risk, estimated at roughly 138 infections per 10,000 exposures. Vaginal sex falls considerably lower, and oral sex is at the bottom of the risk scale.²

For vaginal intercourse specifically, transmission probability varies by direction and setting. In high-income countries, pooled estimates put the female-to-male risk at about 0.04 percent per act and the male-to-female risk at about 0.08 percent per act. In lower-income settings where background viral loads tend to be higher and treatment access lower, those numbers rise substantially.³ These averages obscure important variation, though. The viral load of the partner living with HIV is the dominant factor: each tenfold increase in their blood plasma viral level roughly triples the per-act risk. Self-reported condom use, even inconsistent, cut the per-act risk by about 78 percent in one large study of African serodiscordant couples.⁴

Two things raise the risk sharply beyond these averages: a partner who is in the early acute phase of HIV infection (when viral load peaks before the immune system mounts a response) and the presence of other sexually transmitted infections, which can cause genital inflammation or ulceration. Early-phase infection has been estimated to make someone roughly nine times more infectious than during the chronic asymptomatic phase.³ The practical implication: if you know nothing about your partner’s HIV status, you cannot rule out acute infection, and that uncertainty is exactly the scenario where PEP makes the most sense.

The PEP Regimen and What to Expect

PEP is a 28-day course, no exceptions on the duration. The standard recommended regimen in most current guidelines is a three-drug combination. Older regimens paired tenofovir/emtricitabine (the backbone) with a protease inhibitor or raltegravir, but newer single-tablet regimens have largely replaced them because they are easier to take and people actually finish them. A study comparing a once-daily four-in-one pill (elvitegravir/cobicistat/tenofovir/emtricitabine) with older multi-pill regimens found a 71 percent completion rate for the single tablet, compared to 57 percent for raltegravir-based and just 39 percent for protease-inhibitor-based regimens.⁵

Even newer single-tablet options have pushed completion rates higher. A prospective cohort study in China found that a bictegravir-based single pill achieved a completion rate near 98 percent, compared to about 83 percent for a two-pill dolutegravir-based regimen.⁶ The trend is clear: fewer pills per day and fewer side effects translate directly into more people finishing the course, which is what actually determines whether PEP protects you.

Side effects are common but usually tolerable. The most frequently reported problems are nausea, diarrhea, abdominal discomfort, bloating, fatigue, and headache. In the elvitegravir single-tablet study, about 42 percent of users reported gas or abdominal pain, 38 percent had diarrhea, and 28 percent experienced nausea or fatigue. Most symptoms were mild and did not lead people to stop the medication.⁵

Still, across the broader literature, PEP completion rates remain a problem. A large meta-analysis pooling data from many cohort studies found that only about 58 percent of people who start PEP actually finish the full 28 days. Side effects play a role: roughly 60 percent of PEP users experienced some form of adverse reaction, and about a third of all discontinuations were linked to those reactions.⁷ The lesson: if you start PEP and feel lousy, call the prescribing clinic before you stop taking it. They may be able to manage symptoms or, in some cases, switch you to a better-tolerated regimen. Stopping early means you may not be protected.

Where to Get PEP and Common Barriers

You can get PEP from emergency departments, sexual health clinics, HIV specialty clinics, and many urgent care centers. Some primary care doctors can prescribe it too, though the time-sensitive nature of PEP means you should not wait for a scheduled appointment. If you arrive at an emergency department and the physician is unfamiliar with PEP, you may need to advocate for yourself or ask to speak with an infectious disease specialist on call.

Provider unfamiliarity is a real barrier. A narrative review of PEP underutilization found that comfort with prescribing PEP, including selecting the correct regimen and arranging follow-up, tends to be more limited in acute care settings. Among PEP prescriptions written in emergency departments, nearly 30 percent were for the wrong duration.⁸ That means you could walk out with a prescription for 14 days instead of 28 and think you are covered when you are not. If you are prescribed PEP, confirm that the prescription is for a full 28-day supply. If it is not, push back or get a follow-up appointment within days to extend it.

Cost is another concern. In many countries, PEP is available free or at reduced cost through public health programs, sexual health clinics, or emergency assistance programs. In the United States, most insurance plans cover PEP, and several manufacturer assistance programs and state-funded programs can help cover out-of-pocket costs. If cost is a barrier, a sexual health clinic is often the best first stop because staff there are experienced at navigating coverage.

Stigma keeps some people from seeking PEP at all. Feeling embarrassed to explain the circumstances of exposure, fear of judgment from healthcare providers, or worry about confidentiality can cause people to delay or skip treatment entirely. This is worth naming plainly: a healthcare provider’s job is to give you the medication, not to evaluate your personal life. If the first provider you encounter is unhelpful, try a sexual health clinic where staff deal with PEP requests routinely.

HIV Testing After Exposure

You will get a baseline HIV test at the time you start PEP, primarily to confirm you are not already infected before starting the regimen. This matters because PEP medications are a partial antiretroviral regimen, and taking them when you are already HIV-positive could promote drug resistance. If your baseline test comes back negative, you continue the 28-day course as prescribed.

Follow-up testing is where timing gets tricky. HIV tests detect different things at different time points: some look for the virus itself, some look for both viral proteins and antibodies, and older tests look only for antibodies. Newer lab-based antigen/antibody combination tests can detect HIV infection with a median window period of about 18 days after the virus becomes detectable by the most sensitive molecular tests. Older antibody-only tests take longer, with a median window around 37 days. At the upper end, the 99th percentile of the detection window for the best lab tests is about 44 days.⁹

Rapid point-of-care tests, the kind that give results in minutes, are less sensitive than lab-based tests, particularly in the early weeks after infection. A meta-analysis found their overall sensitivity compared to lab-based tests was about 94 percent, but in high-income settings it dropped to about 86 percent. This means rapid tests can miss a meaningful fraction of very recent infections.¹⁰ For follow-up after a potential exposure, a lab-based fourth-generation antigen/antibody test is the standard. Most guidelines recommend follow-up testing at about four to six weeks after exposure and again at three months for definitive results.

If you are taking PEP, keep in mind that the antiretroviral medications could theoretically delay seroconversion, which is why the follow-up testing timeline usually begins after the 28-day course ends rather than during it.

Recognizing Acute HIV Infection

In roughly 65 to 95 percent of cases, primary HIV infection triggers a set of flu-like symptoms known as acute retroviral syndrome, typically appearing one to four weeks after exposure.¹¹ The symptoms are frustratingly nonspecific, which is exactly why they get missed so often. Fever, sore throat, fatigue, swollen lymph nodes, rash, and muscle aches all overlap with common viral illnesses.

Research from an intensely monitored cohort on two continents found that the symptoms with the strongest association with acute HIV infection (compared to people who tested negative) included nausea, swollen lymph nodes, diarrhea, abdominal pain, and profound fatigue. In African women in the study, pain behind the eyes, fatigue, and unintentional weight loss were among the most distinguishing symptoms. Across the entire cohort, 86 percent of people with acute HIV infection reported at least one symptom during the first three weeks.¹²

None of these symptoms alone means you have HIV. But if you had a high-risk exposure, did not take PEP (or started it late), and then develop a cluster of these symptoms two to four weeks later, get tested immediately with a test that can detect early infection, ideally a lab-based antigen/antibody test or an RNA viral load test. A case report illustrating this scenario described a patient presenting with inflamed tonsils with white patches, tender neck and underarm lymph nodes, a palpable spleen, and an itchy rash on the back, chest, and arms, all of which prompted testing during the window period that confirmed acute infection.¹³ The important point is that early diagnosis, even before standard antibody tests turn positive, matters enormously for both treatment and preventing onward transmission.

What Washing and Douching Will Not Do

A common instinct after unprotected sex is to try to “clean out” the exposure through washing, douching, or using antiseptic products. There is no evidence that any of these measures reduce HIV transmission risk after the fact. For rectal exposure specifically, douching after sex can actually make things worse. Research on rectal douching among men who have sex with men found that some douches cause loss of the surface lining of the rectum, which could increase rather than decrease vulnerability to HIV.¹⁴ Vaginal douching carries similar risks of disrupting protective barriers. The time you spend on hygiene measures would be far better spent getting to a clinic for PEP.

Moving From PEP to PrEP

If you found yourself needing PEP once, there is a reasonable chance the circumstances that led to the exposure could happen again. That is not a moral judgment; it is an epidemiological observation. Data from one clinic found that roughly 13 percent of gay men who obtained PEP were diagnosed with HIV within 12 months of using it, which prompted the clinic to offer pre-exposure prophylaxis to anyone after even a single PEP episode.¹⁵ PrEP is the daily or on-demand medication taken before exposure to prevent HIV. It is highly effective when taken as prescribed, and transitioning from PEP to PrEP is now a well-established pathway in many clinics.

In practice, the transition works by finishing the 28-day PEP course, confirming an HIV-negative result with a follow-up test, and starting PrEP the next day. Some clinics handle this seamlessly within the same care team. A longitudinal study in China found that about 29 percent of PEP users eventually transitioned to PrEP, with a median time gap of about 11 months between PEP use and PrEP initiation. Most who transitioned did so after just one PEP episode.¹⁶ The 11-month gap is striking, because it means months of unprotected vulnerability between finishing PEP and starting PrEP. Ideally that gap should be as close to zero as possible. If you are completing PEP and think you might benefit from ongoing prevention, bring up PrEP with your provider before the 28 days are up.

Preventing Other STIs at the Same Encounter

HIV is not the only infection that can result from unprotected sex, and if you are already seeking care for PEP, it is a natural time to address other sexually transmitted infections. Most clinics will test for gonorrhea, chlamydia, syphilis, and hepatitis B and C at the baseline PEP visit. Treatment for bacterial STIs is straightforward and can start immediately if tests come back positive.

A newer development worth knowing about is doxycycline post-exposure prophylaxis, or doxy-PEP: a single 200 mg dose of the antibiotic doxycycline taken within 72 hours after condomless sex to prevent bacterial STIs. An open-label randomized trial among men who have sex with men and transgender women found that this approach significantly reduced infections with chlamydia, gonorrhea, and syphilis compared to standard care without doxycycline.¹⁷ Doxy-PEP has since been incorporated into some clinical guidelines, particularly for people at high risk of recurrent bacterial STIs. It does nothing against HIV, but when paired with HIV PEP or PrEP, it represents a broader post-exposure prevention strategy that covers multiple infections at once.

Concerns about antibiotic resistance are real and actively being studied. Doxy-PEP is not recommended for everyone, and guidelines are still evolving. But if you are already in a clinic discussing HIV prevention after an unprotected encounter, asking about doxy-PEP is reasonable, especially if you have had bacterial STIs before.

PEP After Sexual Assault

Sexual assault is one of the most urgent contexts for PEP, and it comes with its own set of medical, legal, and emotional considerations. Guidelines for sexual assault care emphasize that PEP should be offered as part of the immediate medical response, alongside emergency contraception for those at risk of pregnancy and comprehensive screening for other infections.¹⁸ If you or someone you know has experienced sexual assault, going to an emergency department or a dedicated sexual assault response center is the best first step. Staff at these centers are trained to provide PEP along with forensic evidence collection and psychosocial support as part of a coordinated response.

The emotional toll of any high-risk exposure can be significant, but for assault survivors the distress is compounded by trauma. Research has found that women living with HIV experience depression, anxiety, and post-traumatic stress at higher rates than both their male counterparts and HIV-unaffected women, and these mental health effects have downstream consequences for treatment adherence and overall well-being.¹⁹ The anxiety of waiting for test results after any potential exposure is real for anyone, and connecting with a counselor or support service during the PEP period and beyond can make the process more manageable.

Expanding Access Through Online and Pharmacy Models

One of the persistent barriers to PEP is simply getting to a provider fast enough, particularly in rural areas or in countries where clinic infrastructure is limited. Modeling work has explored whether making PEP available through online pharmacies could help close this gap. A study projecting the impact of online PrEP and PEP availability in western Kenya estimated that even modest population coverage through an online pharmacy model could avert roughly 12 to 14 percent of HIV infections over a decade.²⁰ These models are still theoretical, but they point toward a future where PEP access is less dependent on physically reaching a clinic within that narrow 72-hour window. Telehealth platforms in several countries already offer PEP consultations, where a clinician evaluates your exposure risk by video call and sends a prescription to a nearby pharmacy. If you cannot get to a clinic in person, searching for telehealth PEP services in your area is worth the few minutes it takes.