DoxyPEP is a single 200 mg dose of doxycycline taken after sex to prevent bacterial STIs. You take it as soon as possible, within 72 hours of oral, vaginal, or anal sex, and you don’t take more than one dose in any 24-hour period. That’s the core protocol, but the details of timing, food, and side effect prevention matter for getting the most out of it.
The Dose and Timing Window
The CDC-recommended dose is 200 mg of doxycycline, which typically means two 100 mg tablets or capsules taken together. Any formulation of doxycycline works. You take it as soon as possible after a sexual encounter, and the outer limit is 72 hours. Sooner is better.
If you have sex multiple times in one day, you still only take one dose. The maximum is 200 mg every 24 hours, regardless of how many partners or encounters you have. If you took a dose yesterday evening and have sex again today, wait until at least 24 hours have passed from the previous dose before taking another.
How to Swallow It Without Stomach Problems
Doxycycline is notorious for causing nausea, stomach pain, and in some cases, irritation or ulceration of the esophagus. You can avoid most of this by following three rules: take it on a full stomach, drink a full glass of water with it, and stay upright for at least one hour afterward. That last point is easy to forget, especially if you’re taking it late at night. Lying down too soon lets the pill sit in your esophagus and cause damage to the lining.
Sun sensitivity is the other common side effect. Doxycycline makes your skin burn more easily under UV exposure. If you’re taking it regularly, use sunscreen or cover up during prolonged time outdoors.
Food and Supplement Interactions
Doxycycline doesn’t absorb well when minerals are present in your stomach. Calcium, magnesium, iron, and zinc all interfere with how much of the drug actually reaches your bloodstream. This means you should separate your dose from dairy products, antacids, and mineral supplements by at least two hours in either direction. A meal is fine and encouraged, just not one centered on milk, cheese, or yogurt.
Which STIs It Prevents
Clinical trials in men who have sex with men and transgender women found that doxyPEP significantly reduced new infections of chlamydia, syphilis, and gonorrhea. Chlamydia and syphilis see the strongest protection. Gonorrhea prevention is less reliable, partly because gonorrhea already has high levels of tetracycline resistance in many areas.
DoxyPEP does not protect against HIV, herpes, HPV, or other viral infections. It’s a targeted tool for bacterial STIs and works best as one layer in a broader prevention strategy.
Who the Guidelines Cover
The 2024 CDC guidelines recommend doxyPEP for men who have sex with men and transgender women who have had a bacterial STI in the past year or are otherwise at increased risk. The evidence base comes primarily from trials in these populations. Data on cisgender women is limited, and the CDC has not extended the same recommendation to that group.
Antibiotic Resistance Concerns
Using an antibiotic intermittently raises legitimate questions about resistance. The picture so far is mixed. For chlamydia and syphilis, resistance to doxycycline has not emerged. Chlamydia cure rates remain above 95% globally, and no confirmed doxycycline-resistant syphilis cases have been reported.
Gonorrhea is a different story. In King County, Washington, after doxyPEP guidelines were introduced in mid-2023, tetracycline resistance in gonorrhea among men who have sex with men jumped from roughly 27% to 70%. High-level resistance climbed from 2% to 65%. The DOXYVAC trial in France saw a similar pattern, with high-level resistance at 36% in the doxyPEP group compared to 13% in controls. This is one reason doxyPEP is less effective against gonorrhea and why ongoing STI screening remains important even if you’re using it.
As for your gut bacteria, a study tracking participants over six months found no significant changes in overall bacterial diversity despite a median of 42 doses. However, the activity of tetracycline resistance genes in the gut did increase meaningfully with more doses. The bacteria were already carrying these resistance genes at baseline in virtually all participants, but doxycycline use ramped up their expression.
Storing Your Supply
Since doxyPEP is taken on an as-needed basis, you may keep a supply on hand for weeks or months. Store it in a cool, dry place away from light. Bathrooms are a poor choice because of humidity. A bedroom drawer or a cool closet works better. Tablets and capsules are the most stable drug forms over time, but keep the bottle tightly capped and check expiration dates periodically.