During the recently concluded Philippine Society for Microbiology and Infectious Diseases Annual Convention last Nov. 18-20, 2025, I was asked to engage in a friendly debate with colleagues. The issue was whether post-exposure prophylaxis for sexually transmitted infections (STIs) using doxycycline, an antibiotic with activity against several STIs, was appropriate for the Philippine setting. Most Filipinos are familiar with doxycycline as the go-to prophylaxis against leptospirosis after wading in flood waters. Several studies have now shown that it is also effective in decreasing STIs if taken shortly after a high-risk sexual encounter.

Doxycycline is used as an STI treatment primarily against chlamydia infection. Chlamydia is a hard-to-grow bacterium that lives inside human cells and can cause urethritis (inflammation of the urethra, which is the passageway of urine from the bladder to the outside) in both males and females. While it can cause some annoying symptoms in males, chlamydia is particularly dangerous for females since it can cause pelvic inflammatory disease (PID), which is a leading cause of infertility and can progress to sepsis and death. Treatment of chlamydia infection usually entails taking doxycycline 100mg twice a day for seven days. Post-exposure prophylaxis with doxycycline uses a single high dose (two 200 mg tablets) regimen within 72 hours of a high-risk unprotected sexual encounter. Doxycycline post-exposure prophylaxis, or DoxyPEP for short, has been shown in several landmark trials to be effective in decreasing subsequent chlamydia infections in men who have sex with men and transgender women. In addition, it also decreased the rates of gonorrhea and syphilis infection during the trials.

Gonorrhea is an STI caused by the bacterium Neisseria gonorrhea. It causes an exuberant pus-like urethral discharge a few days after exposure and can be quite distressing, especially in men. It is less obvious in women, as the discharge can be mistaken to be part of normal vaginal discharge. In combination with chlamydia, gonorrhea can increase the risk of PID. In recent years, local doxycycline resistance in gonorrhea has climbed to such high levels that doxycycline is no longer considered a viable treatment for gonorrhea.

Syphilis is caused by the bacterium Treponema pallidum and can cause myriad complications. It can cause a painless ulcer initially and can later produce a whole-body rash that involves the palms and soles. Untreated syphilis can lead to nerve damage, brain damage, and destruction of blood vessels and heart valves. It can also cause destructive bone and skin lesions. Very early syphilis can be treated with doxycycline. Intramuscular or intravenous penicillin, however, is the preferred option for the treatment of established syphilis infection.

While doxycycline is not a first-line treatment for either syphilis or gonorrhea, it is known to have some activity against both. When used as prophylaxis, the antibiotic can be more effective because there is not yet an established infection, and the amount of bacteria present is small. One valid concern of using DoxyPEP, however, is the development of resistance not just in the target bacteria, but also among bystander bacteria like Staphylococcus aureus and Escherichia coli.

To spice things up, I was asked to take the “against” DoxyPEP side along with a younger infectious diseases physician, Dr. Jan Jorge Francisco. Given my longstanding advocacy against HIV and other STIs, I am naturally for the use of interventions that can potentially decrease the risk of STIs. In addition, other STIs can increase the risk of acquisition and transmission of HIV, and so DoxyPEP can help decrease new cases of HIV. On the “for” side was Dr. Regina Berba, a staunch antimicrobial resistance advocate and a strong proponent of rational antimicrobial use, along with Dr. Bryan Lim, another junior infectious diseases colleague from Cebu. Needless to say, neither Dr. Regina nor I were within our comfort zone since we were asked to defend positions which would have been the opposite of our advocacies in real life.

The format of the debate had each side presenting their opening statements, followed by the first rebuttal, a second rebuttal, and a final statement. The effectiveness of the argument was determined by doing an initial audience poll on their position (for, undecided, or against), followed by a post-debate poll to see if the positions had changed. The team with the highest proportion of the audience switching to their position was considered the winner. The initial poll showed a nearly even distribution, with 1/3 of the audience advocating for the use of DoxyPEP, 1/3 against, and 1/3 undecided.

Dr. Regina opened the debate by citing several randomized placebo-controlled trials (RCTs) done in the US, Canada, and Australia as strong proof that DoxyPEP works. In those studies, DoxyPEP reduced the risk of all three STIs (gonorrhea, chlamydia, and syphilis) by nearly 2/3 in men who have sex with men as well as transgender women. Based on these studies, she recommended that the Philippines adopt the use of DoxyPEP in these populations.

Dr. Jan presented our side, and he highlighted the very real risk of increasing antibiotic resistance from the use of DoxyPEP. Given our already high resistance of gonorrhea to tetracyclines, including doxycycline, it is doubtful that it would make a big difference in preventing this disease. Moreover, the use of DoxyPEP can exacerbate increasing resistance to tetracyclines in medically important bacteria such as Staphylococcus aureus and E. coli.

Dr. Bryan Lim was up next, with a rebuttal of our position, where he correctly argued that theoretical risks are not equivalent to known, real-world benefits. He stated that case reports of gonorrhea resistance cannot offset positive results from meticulously performed RCTs. I was up next with my rebuttal, and I presented data showing that DoxyPEP failed in RCTs in France, where gonorrhea resistance to doxycycline was high, and in Kenya, where it failed to show any benefit among cisgender women. Since Kenya is a resource-limited setting like ours, and our high gonorrhea resistance to tetracyclines is similar to that of France, I asserted that similar studies in our setting would likely fail. After the final statement of each group, the second poll was initiated. It showed that nearly everyone who was undecided had been convinced by our side that DoxyPEP should not be used in our setting.

The debate was a great academic exercise. We were able to defend the position assigned to us, and the other team was able to do the same. In truth, I would probably still consider using DoxyPEP in a patient if he or she requests it, depending on individual circumstances. It is possible to appreciate the pros and cons of an intervention, whatever your personal beliefs may be, and decisions should be made in a nuanced manner. This debate was a showcase for how to interpret evidence without bias and being open to what science says, regardless of one’s personal opinions. I wish our everyday discourses in real life and on social media could be like these scientific discussions. People should be able to intelligently discuss the benefits and advantages of an intervention and come away with a consensus for the good of their patients in a rational manner. If we are able to do this consistently, then everyone wins.