Doxy-PEP (sometimes mistakenly called “doxy prep”) is a strategy for preventing bacterial sexually transmitted infections by taking a single dose of the antibiotic doxycycline after sex. “PEP” stands for post-exposure prophylaxis, meaning you take it after a potential exposure rather than before. The CDC issued formal clinical guidelines for doxy-PEP in 2024, making it the first recommended antibiotic-based tool for STI prevention.
How Doxy-PEP Works
Doxycycline is a tetracycline antibiotic that has been used for decades to treat infections like chlamydia and syphilis. The idea behind doxy-PEP is simple: by taking a dose soon after sex, the antibiotic can kill bacteria before they establish an infection. It targets bacterial STIs specifically. It does nothing against viral infections like HIV, herpes, or HPV.
Clinical trials have shown it is particularly effective at preventing chlamydia and syphilis. Its effectiveness against gonorrhea is less consistent, partly because gonorrhea bacteria have developed significant resistance to tetracycline antibiotics over the years.
Who It’s Recommended For
The CDC currently recommends doxy-PEP for gay and bisexual men and transgender women who have had a bacterial STI in the past year. These groups showed the clearest benefit in clinical trials. The recommendation is built around shared decision-making, meaning your provider discusses the option with you and writes a prescription you can keep on hand and use as needed.
There isn’t yet strong enough evidence to recommend doxy-PEP broadly for cisgender women or other populations. This isn’t because it’s assumed to be harmful for those groups, but because the clinical trial data so far comes primarily from men who have sex with men and transgender women.
Dosage and Timing
The dose is 200 mg of doxycycline, taken as soon as possible after oral, anal, or vaginal sex. The outer limit is 72 hours, but sooner is better. You should not take more than one 200 mg dose in any 24-hour period, regardless of how many sexual encounters you have that day.
This makes it different from HIV PrEP, which is taken on a daily or event-driven schedule before sex. Doxy-PEP is reactive: you take it after the fact, only when you need it.
Side Effects
Doxycycline is a well-known medication with a long safety track record. The most common side effects at this dose include nausea, upset stomach, and increased sun sensitivity. Taking it with food and water helps reduce stomach irritation. Because you’re taking it intermittently rather than daily, side effects tend to be mild and short-lived for most people.
Doxycycline can also reduce the effectiveness of hormonal birth control in some cases, and it shouldn’t be taken during pregnancy. If you’re on other medications, it’s worth confirming with your provider that there are no interactions.
Getting a Prescription
Doxy-PEP requires a prescription, but the process is straightforward. Your provider can write one without special authorizations or extra paperwork. Prescriptions are typically written for a one- to three-month supply based on your anticipated use, so you have pills available when you need them. Doxycycline is a generic antibiotic and is generally inexpensive. In California, for example, it’s covered through both the state’s AIDS Drug Assistance Program and PrEP Assistance Program formularies. Many insurance plans cover it, and the out-of-pocket cost without insurance is usually modest compared to brand-name medications.
The Antibiotic Resistance Question
The biggest concern around doxy-PEP is whether widespread use could accelerate antibiotic resistance. This is not a hypothetical worry. Data from a large study published in Clinical Infectious Diseases found that among men who have sex with men, tetracycline resistance in gonorrhea rose from a stable average of 27% between 2017 and early 2023 to 70% by mid-2024. High-level resistance jumped from 2% to 65% over a similar period.
The relationship to doxy-PEP specifically is nuanced. Taking more than three doses per month was associated with a 42% higher likelihood of carrying tetracycline-resistant gonorrhea compared to non-users. However, any use of doxy-PEP (without that frequency threshold) was not significantly linked to resistance. The same study found that doxy-PEP users were more likely to carry tetracycline-resistant staph bacteria and group A strep bacteria on their bodies, even though overall staph colonization was actually lower among users.
This means doxy-PEP is most effective and poses the least resistance risk when used selectively rather than as a daily habit. The fact that it works well against syphilis and chlamydia but less reliably against gonorrhea already reflects the resistance patterns in play.
What Doxy-PEP Does Not Replace
Doxy-PEP is one layer of protection, not a substitute for other strategies. It does not prevent HIV, herpes, HPV, or hepatitis. Condoms remain the only method that reduces risk across both bacterial and viral STIs simultaneously. People using doxy-PEP are still advised to get regular STI screening, since the antibiotic won’t catch every exposure and gonorrhea in particular may break through.
If you’re already on HIV PrEP, doxy-PEP can be used alongside it. The two medications address completely different infections and don’t interfere with each other.