Doxycycline is a broad-spectrum, tetracycline antibiotic that may be taken within 72 hours of unprotected sex to help prevent common bacterial STDs (sexually transmitted diseases), such as syphilis, chlamydia, and gonorrhea.

This doxycycline treatment, known as the morning-after-pill for STIs or Doxy PEP is recommended by the CDC (Centers for Disease Control and Prevention) for gay and bisexual men and transgender women who have had an STD in the past year and are at high risk of reinfection. STDs are also known as STIs (sexually transmitted infections).

There is not enough evidence to determine its effectiveness in other populations, such as heterosexual men and women, or people who have never had an STD.

Doxycycline for STD dosage

The recommended dosage is doxycycline 200 mg taken as a single dose.

• Doses should be taken as soon after sex as possible, but no later than 72 hours.
• Do not take more than 200 mg of doxycycline per 24 hours.
• Take doxycycline exactly as prescribed by your healthcare provider and only for its intended purpose. Do not share your doxycycline.

How should I take doxycycline for STDs?

Most doxycycline brands, except the Oracea brand, can be taken with milk or food, which can help if the medication upsets your stomach. The Oracea brand should be taken on an empty stomach—at least 1 hour before or 2 hours after a meal—since food can reduce its absorption and effectiveness.

• Drink a full glass of water and avoid lying down for 1 hour after taking doxycycline to prevent esophagitis (an irritation to the lining of your esophagus).
• Ask your doctor if any medications you take interact with doxycycline. There have been no clinically relevant interactions between doxycycline and gender-affirming hormonal therapy reported.

What are the side effects of doxy PEP?

The most common side effects of doxycycline taken for STD prevention include:

• Gastrointestinal effects such as diarrhea, nausea, vomiting, indigestion, bloating, loss of appetite, esophagitis, or abdominal pain
• A skin rash
• Sensitivity to light (photosensitivity)
• The development of resistant bacteria.

How effective is doxycycline for STDs?

Research to date has shown the one-off doxycycline dose is very effective at preventing bacterial sexually transmitted infections, specifically syphilis, chlamydia, and gonorrhea.

The CDC recommendations were based on the results of 4 trials, mainly conducted in MSM (men who have sex with men) and TGW (transgender women):

• A 70% reduced risk for acquiring chlamydia and a 73% reduced risk for acquiring syphilis over 10 months was reported for 232 HIV-negative MSM and TGW who took doxycycline 200mg within 72 hours of unprotected oral or anal sex. There was no difference in the rate of gonorrhea infections. These participants were also taking Truvada for HIV PrEP (IPERGAY Study; open-label extension phase).
• Significant differences in relative risk of contracting gonorrhea, chlamydia, and early syphilis were reported for another cohort of 501 MSM and TGW assigned doxycycline 200mg within 24 hours of unprotected sex who either had HIV or were taking HIV PrEP (DoxyPep; randomized open-label trial, 2022).
• A randomized trial that assigned 332 MSM to doxy PEP (200 mg within 24 hours) and 170 MSM to no doxy PEP was stopped early because of the significant effect the treatment arm (doxy PEP) had compared with the no-treatment arm. (ANRS DOXYVAC; randomized, 2022).
• No significant differences in bacterial STIs were reported in 449 cisgender Kenyan women assigned doxycycline 200mg within 72 hours of unprotected sex. However, hair studies detected doxycycline in only 29% of participants, suggesting nonadherence or biological differences may be a factor in the result.

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How often should you test for STDs?

Many STDs often show no symptoms, which is why the CDC recommends regular screening for all sexually active individuals. The recommended screening frequencies for specific infections are:

HIV:

• At least once a year for everyone aged 13-64.

Gonorrhea and chlamydia:

• Annually for sexually active women under 25.
• Annually for women 25 and older with risk factors (e.g., new or multiple partners).

During pregnancy:

• Early testing for syphilis, HIV, hepatitis B, and C.
• Additional testing for chlamydia and gonorrhea if at risk.

Gay/Bisexual Men, MSM, and TGW:

• Syphilis, chlamydia, and gonorrhea at least once a year; every 3-6 months if there are multiple or anonymous partners or on HIV PrEP.
• HIV at least once a year; more frequently if needed.
• Hepatitis C annually if living with HIV.

Injection Drug Users:

• HIV at least once a year.

How do you test for STDs?

STD testing involves a sample of body fluid to check for infections and may include swab tests, blood tests, or urine tests. Your healthcare provider will select the right test for you based on your sexual behavior, symptoms, history of STDs, and overall risk assessment.

• Different tests may be needed based on whether you've had vaginal, anal, or oral sex. For example, oral swab tests are appropriate for oral sex-related risks.
• If you have symptoms like sores or unusual discharge, a swab test from the affected area may be necessary.

Swab tests

• Purpose: Detects infections in the genital, throat, anus, or sores.
• Commonly detects: Chlamydia, gonorrhea.
• Procedure: Samples are taken from the affected area using a sterile swab.
• Advantages: Higher accuracy for oral and anal infections compared to urine tests.

Blood tests

• Purpose: Identifies diseases such as HIV, syphilis, and hepatitis.
• Procedure: Blood is drawn from a vein and analyzed in a lab.
• Advantages: Can detect infections even without symptoms and identify antibodies.

Urine tests

• Purpose: Detects diseases like chlamydia and gonorrhea.
• Procedure: A urine sample is collected and analyzed.
• Advantages: Non-invasive and easy to perform.
• Limitations: May not detect infections in the throat or rectum, which require swab tests for accurate diagnosis.

References

• Preexposure prophylaxis for the prevention of HIV infection in the United States. 2021 Update. https://www.cdc.gov/hiv/pdf/risk/prep/cdc-hiv-prep-guidelines-2021.pdf
• STD Testing. What’s right for you? Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/sexually-transmitted-diseases-stds/in-depth/std-testing/art-20046019
• Getting tested for STIs. 04/2024. CDC. https://www.cdc.gov/sti/testing/index.html#:~:text=All%20sexually%20active%20women%20younger,partner%20who%20has%20an%20STI.
• STI Testing (STD Testing). Cleveland Clinic. https://my.clevelandclinic.org/health/diagnostics/std-testing
• Doxycycline 04/2024. Drugs.com https://www.drugs.com/doxycycline.html
• Bachmann LH, Barbee LA, Chan P, et al. CDC Clinical Guidelines on the Use of Doxycycline Postexposure Prophylaxis for Bacterial Sexually Transmitted Infection Prevention, United States, 2024. MMWR Recomm Rep 2024;73(No. RR-2):1–8. DOI: http://dx.doi.org/10.15585/mmwr.rr7302a1 https://www.cdc.gov/mmwr/volumes/73/rr/rr7302a1.htm?_cid=rr7302a1_w#suggestedcitation

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