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Does doxycycline reduce the incidence of STIs?

A recent study published in The Lancet evaluated whether doxycycline reduces the incidence of STIs among men who have sex with men.

While the number of new cases of AIDS and other sexually transmitted infections (STIs) has lowered in the last decades, there has still been a global increase in the incidence rates of STIs. Men who have sex with men (MSM) without condoms are the most vulnerable group for STIs, in the United States, bacterial sexually transmitted infections such as gonorrhea, chlamydia and syphilis, have become more and more prevalent.

What is post-exposure prophylaxis?

Post-exposure prophylaxis (PEP) is the process of taking antiretroviral medications after an individual has been potentially exposed to HIV, to prevent the person from becoming infected. Although pre-exposure prophylaxis (PrEP) with an oral combination of medicines has proven effective in preventing new cases of HIV/AIDS among MSM, bacterial sexually transmitted infections are still at high rates. So far, no effective vaccines against such infections exist, and condom use remains the cornerstone of prevention.

A well-known antibiotic, doxycycline, has been successfully used for post-exposure prophylaxis (PEP) of Lyme disease and leptospirosis, and its role in preventing STIs is being investigated. Recently, a French study published in the journal The Lancet evaluated the efficacy of PEP with doxycycline in reducing the incidence of bacterial sexually transmitted infections among MSMs that have a high risk.

Does doxycycline reduce the incidence of bacterial STIs?

For the study, the researchers included MSM who were HIV-negative, at least 18 years or older, and at a high risk of acquiring HIV (having condomless anal sex with at least two different partners during the past six months). The researchers randomly divided these men into two groups: an intervention group and a control group.

The intervention group received PrEP with tenofovir disoproxil fumarate plus emtricitabine and PEP with doxycycline.  The other group did not receive doxycycline PEP. Participants in the intervention group took 200 mg doxycycline within 24 hours after having unprotected sex, and study visits were scheduled every two months. At the enrollment visit and every two months after, the researchers tested the participants thoroughly for the three STIs: syphilis, chlamydia and gonorrhea. The follow-up period lasted for approximately eight months.

Doxycycline PEP may be effective in preventing chlamydia and syphilis

From the 232 individuals selected for the study, 116 received doxycycline PEP and 116 did not. A total of 78 participants presented with a new STI during follow-up (28 in PEP group and 45 in the no-PEP group).

In general, the occurrence of a first STI in the PEP group was lower than in the no-PEP group. When analyzing the incidence of gonorrhea, 47 presented a new episode of the disease during follow-up, which was not statistically significant. Twenty-eight participants presented with chlamydia in follow-up, which shows a significant difference in incidence between these two groups. Thirteen individuals presented with syphilis in follow-up, showing a significant difference between the two groups. Finally, the incidence of all STIs (first and other episodes) during the study was 38 in the PEP group and 64 in the no-PEP group. No HIV seroconversion was reported during the study.

The study also shows that sexual practices remained similar between study groups during the study period and that the frequency of serious adverse events did not differ significantly between them. In general, it was verified that, among MSM using PrEP with tenofovir disoproxil fumarate plus emtricitabine for HIV prevention, the use of doxycycline PEP following condomless sexual activity is associated with a significant decrease in the occurrence of new bacterial STIs. However, the short duration of follow-up might have increased the probability of an exaggerated estimate of efficacy of this prophylactic approach.

Due to the well-known gonorrhea antibiotic resistance, there was no change in the occurrence of a new episode of gonorrhea in the group using doxycycline PEP. On the other hand, there was a sharp decrease in new episodes of chlamydia and syphilis with doxycycline PEP.

Further studies are needed before PEP can be recommended to high-risk individuals

The authors emphasize the importance of investigating the possibility of antibiotic resistance that the PEP approach could have in the future, especially with respect to antibiotic-resistant strains of gonorrhea. Also, they suggest that studies investigating bacterial STIs with longer follow-up periods are necessary before recommending the widespread use of PEP in high-risk individuals.

Written by Gustavo Caetano, B.Sc., M.Sc.

Reference: Molina J-M, Charreau I, Chidiac C, Pialoux G, Cua E, Delaugerre C, Capitant C, Rojas-Castro D, Fonsart J, Bercot B, Bébéar C, Cotte L, Robineau O, Raffi F, Charbonneau P, Aslan A, Chas J, Niedbalski L, Spire B, Sagaon-Teyssier L, Carette D, Le Mestre S, Doré V, Meyer L. Post-exposure prophylaxis with doxycycline to prevent sexually transmitted infections in men who have sex with men: an open-label randomised substudy of the ANRS IPERGAY trial. Lancet. 2018, v. 18, 308-317.

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