What Is PEP? HIV Post-Exposure Prophylaxis Explained

Waking up worried that a condom slipped, or after a needle injury at work, is frightening. You don't have to wait and wonder: if it happened in the last three days, there's an emergency option that can help. This guide explains what PEP is, when it's recommended, why the 72-hour limit matters, and the tests you'll need afterwards.

What Is PEP?

PEP stands for post-exposure prophylaxis: antiretroviral (HIV) medicines taken after a possible exposure to HIV to stop the virus establishing an infection.

• Prevention, not a cure. PEP is for someone who doesn't have HIV and may just have been exposed. It isn't a treatment for HIV.
• A 28-day course. The medicines are taken every day for 28 days.
• Prescribed after an assessment. A doctor checks the exposure, your health and your other medicines before choosing the regimen.
• For emergencies. It's a backup when something unexpected happens, not a regular method of prevention, and it doesn't guarantee protection.

How Does PEP Work?

When HIV enters the body through a mucous membrane or broken skin, it first infects immune cells near the point of entry, then spreads to the lymph nodes over the following days. PEP combines medicines that block different steps the virus needs to copy itself. Taken early, they can stop the virus spreading before it sets up a permanent infection. That's why speed matters, and why the course has to be taken every day without gaps.

PEP vs PrEP: What's the Difference?

Both use HIV medicines, but they do different jobs and can't be swapped for each other. For more on the daily option, see what is PrEP?

When Is PEP Recommended?

A doctor decides based on the type of exposure, what's known about the other person, and how much time has passed. Get assessed urgently after:

• Anal or vaginal sex without a condom, or a condom that split or came off, with a partner who has HIV and isn't on effective treatment, or whose HIV status you don't know.
• Sexual assault. Hospitals can also offer emergency contraception and tests for other infections.
• Sharing needles or other injecting equipment.
• A needlestick or cut from a needle or sharp instrument with blood on it, for example at work in healthcare or a lab.
• Blood or other infectious fluid splashing into an open wound, or into the eyes, nose or mouth.

When PEP usually isn't needed

• Your partner has HIV but has an undetectable viral load on treatment: they don't pass HIV on through sex.
• Oral sex, which carries a very low risk; PEP is rarely recommended for it.
• Contact with intact skin, or with saliva, sweat, tears or urine with no blood in them.
• Everyday contact: sharing offices, pools, cups or cutlery with someone living with HIV.

If you're unsure whether something counts, it's better to ask a doctor quickly than to wait.

Why the 72-Hour Window Matters

PEP must start within 72 hours (3 days) of the exposure, and the earlier the better, ideally within hours. The sooner the medicines are in your blood, the better their chance of stopping the virus. Guidelines generally don't recommend starting PEP after 72 hours, because there's little evidence that it works then.

• Don't wait until the third day to get help.
• Go even if you don't know the other person's HIV status.
• If you can safely ask the other person to test, or to share their viral load, that helps the doctor, but don't delay starting PEP while you find out.

What Happens When You Ask for PEP?

The conversation is confidential. The doctor will ask:

• When the exposure happened, to the hour if you can.
• What happened: the type of sex or injury, and whether a condom was used or broke.
• What's known about the other person's HIV status and treatment.
• Your other medicines, health conditions and allergies.

Tests before you start

• A baseline HIV test, to check you don't already have HIV.
• Kidney function, and sometimes liver function, because some PEP medicines are cleared by the kidneys.
• Hepatitis B and C. Hepatitis B status matters because tenofovir, used in most PEP regimens, also acts on hepatitis B. If you're not immune, the doctor may start hepatitis B vaccination.
• A pregnancy test where relevant.
• Tests for other STIs after a sexual exposure.

HIV Testing Before and After PEP

A negative HIV test at the start shows you didn't already have HIV. It can't tell you about the exposure you're there for, because that's still inside the window period, the time before a test can detect a new infection.

So follow-up tests are booked after the course. CDC recommends a laboratory 4th generation (antigen/antibody) test at 4 to 6 weeks and again at 12 weeks after the exposure. UK guidance (BASHH) uses a single 4th generation test at least 45 days after the last dose of PEP. In some situations a doctor will add an HIV RNA test. If infection did happen despite PEP, the medicines can delay the point at which tests turn positive, which is why the usual window periods for someone who hasn't taken PEP don't apply. Follow your doctor's dates rather than a generic calculator, and go to every follow-up test even if you feel well.

After a sexual exposure, PEP doesn't treat other STIs, and tests for syphilis, chlamydia and gonorrhoea follow their own timings: the STI test window calculator gives the dates for those. For how the different HIV tests work, see what is HIV?

PEP Medicines and Regimens

Today's PEP is far simpler and better tolerated than older regimens. WHO and CDC recommend a combination of three HIV medicines, usually as one to three tablets a day. The preferred combinations are built from:

• Dolutegravir or bictegravir, integrase inhibitors.
• Tenofovir (TDF or TAF), with emtricitabine (FTC) or lamivudine (3TC).

The doctor chooses the regimen around your kidney function, pregnancy or plans for pregnancy, other medicines and supplements, and any allergies. Never take PEP medicines from a friend or buy them online without a prescription: the dose, the combination and the follow-up all need a doctor.

How to Complete the 28-Day Course

Taking PEP exactly as prescribed for the full 28 days is one of the biggest factors in whether it works. Missed doses or stopping early give the virus a chance to multiply.

• Take it at the same time every day, with a daily phone alarm.
• Carry a day's dose with you if you'll be out late or travelling.
• If you miss a dose, contact the clinic or pharmacist that prescribed it for advice on your particular regimen.
• Don't stop early, even if you feel fine, have mild side effects or hear new information about the other person, without talking to your doctor first.

PEP Side Effects and Drug Interactions

Most people have no side effects or only mild ones in the first week or two, which usually settle: nausea, loose stools, headache and tiredness.

Some medicines and supplements interact with PEP. Antacids, indigestion remedies and supplements containing calcium, iron, magnesium or zinc, for example, can reduce how much dolutegravir or bictegravir is absorbed, so the timing may need adjusting. Tell the doctor about everything you take, including blood pressure, diabetes and mental health medicines, herbal remedies, alcohol and recreational drugs.

After PEP: Moving On to PrEP

Finishing the 28 days is a milestone, but care isn't over until your final HIV test is negative. If you're likely to be exposed again, ask about PrEP. CDC says people at ongoing risk can move straight from PEP to PrEP without a gap once an HIV test confirms they're negative. Your doctor can discuss it at your follow-up appointment.

How much does PEP cost?

The cost depends on where you go and what's included. It usually covers the doctor's assessment, the baseline tests (HIV, kidney function, hepatitis B and C), the 28-day supply of medicines, and the follow-up visits and tests. Private hospitals and clinics in Bangkok are generally quicker and charge more; public hospitals usually cost less but can involve longer waits. Some international health insurance policies cover PEP, so check yours, but don't let paperwork delay starting it.

PEP and Testing After PEP in Bangkok

If the exposure was in the last 72 hours, go to a hospital emergency department or a clinic that provides PEP in Bangkok today. Emergency departments are open around the clock, which matters at night or at weekends.

Once you've started PEP, Ovio can help with the testing that follows. Our at-home STI testing brings a doctor to your home, condo, hotel or office in Bangkok at a time that fits your follow-up dates. The laboratory 4th generation HIV test is ฿1,290, and an HIV PCR (RNA) test is ฿5,990 where your doctor recommends one. For the other infections that can follow a sexual exposure, the Ovio Essential package (฿5,990) covers HIV, syphilis, hepatitis B and C and an 18-infection PCR panel.

Frequently Asked Questions

How soon do I need to start PEP?

As soon as possible, and within 72 hours of the exposure. The earlier you start, the better it works, so go to a hospital or clinic straight away rather than waiting for the third day.

Is PEP 100% effective?

No. PEP greatly reduces the chance of HIV taking hold, especially when started early and taken for the full 28 days, but it doesn't guarantee protection. That's why follow-up HIV tests are part of the course.

Can I have an HIV test straight after an exposure instead?

A test straight away only shows whether you already had HIV. No test can detect an infection from the last few days, so if the exposure was within 72 hours, ask about PEP first.

When should I test for HIV after PEP?

Follow your doctor's schedule. CDC recommends a 4th generation test at 4 to 6 weeks and again at 12 weeks after the exposure; UK guidance uses one test at least 45 days after the last dose.

Can I drink alcohol while taking PEP?

Alcohol doesn't stop PEP working, but drinking heavily makes it easier to miss or vomit up a dose. Tell your doctor about alcohol and any recreational drugs so they can check for interactions.