Clinically reviewed by Dr. Pipat Finn, M.D., Senior Doctor, Doctor Pattaya.
Last reviewed: August 2026
PEP, post-exposure prophylaxis, must be started within 72 hours of a potential HIV exposure. The medications begin circulating in your body after the first dose, but it takes consistent daily dosing over several days to build drug levels high enough to suppress the virus. You must complete the full 28-day course for PEP to work. A single missed dose does not automatically mean the treatment has failed, but you should contact your doctor the same day it happens.
It is early morning in Pattaya. Something happened last night that you did not plan for. Maybe a condom broke. Maybe a situation got away from you. Now you are sitting in a hotel room somewhere between Pratumnak and the beach, phone in hand, trying to work out what comes next. That feeling is very familiar to us. We see people in exactly this position regularly, and the most important thing we can tell you right now is this: the clock is running, and getting in front of a doctor today matters far more than finishing this article first.
PEP works well when started quickly and taken correctly. But "taken correctly" is the part most people are fuzzy on. What happens if you miss a dose on day 12 because you were out on a day trip to Ko Lan? What if you vomited an hour after swallowing the tablet? This article gives you straight answers to those questions, whether you are about to walk into a clinic or already halfway through your course.
The 72-Hour Window and Why Timing Is Everything
HIV does not spread through your body instantly. In the first hours after exposure, the virus is replicating locally at the site of contact. Over the next day or two, it starts moving toward nearby lymph nodes. Once it reaches your lymphatic system and bloodstream, the window for interception closes.
Starting PEP within 24 hours gives the medication the best chance of catching the virus early. Starting at 48 hours is still worthwhile. The 72-hour mark is the outer limit, and after that, PEP is not recommended. If you are already past 48 hours and wondering whether it is still worth coming in, come in now. Let a doctor make that call, not a Google search.
How Long Does PEP Take to Work?
Your first dose starts circulating within a few hours of swallowing it. But one dose is not enough. It takes consistent daily dosing over several days to build drug levels high enough to suppress viral replication. This is the entire reason the 28-day course exists.
PEP does not flip a switch. Think of it as building a wall, one brick every 24 hours. Miss a brick and you leave a gap. Stop building early and the wall never gets finished. Gaps in that drug level, from missed doses or stopping the course short, are where the risk lives.
Which Medications Are Used and Why Three Drugs Matter
Modern PEP uses three antiretroviral drugs. Two drugs give HIV more room to mutate and survive. Three drugs, each targeting a different point in the virus’s replication process, make that resistance far less likely.
Current guidelines from the WHO and CDC recommend regimens built around integrase inhibitors, most commonly dolutegravir or bictegravir, paired with tenofovir-based medications. These are well-tolerated by most people. At Doctor Pattaya, we prescribe current guideline-recommended regimens and check your baseline health before starting, including kidney function and hepatitis B status.
What Happens If You Miss a PEP Dose
This is the question we get most often, usually from someone already anxious, messaging at midnight. Here is the honest answer.
If you remember within 24 hours, take the missed dose straight away and continue your normal schedule. Do not double up. If more than 24 hours have passed, do not take two tablets at once. Contact your doctor before taking anything. Our STD treatment page covers what to do if you miss a dose and when to come back in.
If you have missed two or more doses in a row, call us the same day. That decision needs a doctor, not a guess. Repeated gaps in your drug levels can compromise the entire course.
What If You Vomit After Taking PEP
Nausea is the most common PEP side effect, and vomiting happens, especially in the first week. This is not a trivial issue.
If you vomited within 30 to 60 minutes of your dose, the medication may not have absorbed fully. You might need to redose, but this depends on which regimen you are on, so call your doctor first. If it was more than two hours after the tablet, absorption is usually complete and you do not need to repeat it. Doctor Pattaya is open around the clock, so you can call and get a straight answer without waiting until morning.
Taking PEP with food often cuts nausea significantly. If side effects are making it hard to stay on schedule, tell us early. There are practical options and we would rather sort it out at day three than hear about it at day ten.
When to Test for HIV After Finishing PEP
A lot of people finish the 28-day course, take a test the next day, get a negative result, and assume it is over. That is not the right approach.
PEP suppresses viral activity during treatment, which can delay when the virus shows up on a test. Testing too early after finishing the course can give a misleading result. The recommended schedule is a 4th-generation HIV test at four to six weeks post-exposure, with a confirmatory test at 12 weeks. A negative result at 12 weeks is considered conclusive under current guidelines.
If you are leaving Pattaya before the 12-week mark, we can map out your testing timeline at your follow-up and help you coordinate care back home.
Getting PEP in Pattaya: What Your First Appointment Looks Like
Walk in, or call ahead if you want, but you do not need an appointment. We are open 24 hours and we start people on PEP every week. The first visit is straightforward.
We take a brief history of the exposure, check your current health, and run baseline blood tests covering kidney function, liver function, hepatitis B, hepatitis C, and an initial HIV test. We also check for concurrent STIs, since the same exposure event often involves more than one risk. The whole thing takes around 30 to 45 minutes. You leave with your first dose the same visit. We see tourists from across Pattaya, Jomtien, and Pratumnak, as well as long-term expats and local residents. Hotel visits are available if getting to the clinic is not possible.
When to Consider Switching From PEP to PrEP
PEP is an emergency measure. It is not built for repeated use. If you have needed it more than once or twice in a year, that pattern is worth paying attention to.
PrEP, pre-exposure prophylaxis, is a daily medication taken before potential exposure. It substantially reduces HIV transmission risk and is a much better fit for anyone with ongoing exposure risk. If we have seen you for PEP treatment before, or if you recognise that your risk is not a one-off, that conversation is worth having. We can assess whether PrEP suits you at the same visit, no separate referral needed.
If you need PEP now or have questions about a missed dose, Doctor Pattaya is open 24 hours, seven days a week in South Pattaya. We start patients on PEP the same visit, run baseline testing on-site, and offer follow-up care through your full 28-day course. Hotel visits are available across Pattaya, Jomtien, and Naklua. Walk in or contact us directly. Do not wait if the clock is already running.
Frequently Asked Questions
Dr. Pipat Finn is the senior doctor at Doctor Pattaya Takecare Clinic, a 24/7 walk-in medical practice on South Pattaya Road, Chonburi, Thailand. They provide primary care, travel medicine, sexual health, vaccination, minor surgery, IV therapy, and emergency outpatient services for tourists, expatriates, and Pattaya residents. Languages: English and Thai. WhatsApp consultations: +66 95 073 5550. Call: +66 81 668 5557.
