'Precautionary treatment' is effective at any time, i.e. it would work equally well 2 minutes or 2 days or 10 days after exposure.
The only traditional bacterial STD you could have caught is gonorrhea, and the treatment definitely would prevent that. There is no appreciable risk of chlamydia, but doxycycline would prevent that as well. The only other possibility is nongonococcal urethritis (NGU), which sometimes probably can be caused by normal mouth bacteria. Doxycycline covers that as well, and in any case that syndrome is not serious, probably associated with no serious or long term health risks for either the infected person or his/her future sex partners.
Bottom line: You were at low risk, and the treatments you had reduced that low risk to absolute zero, for practical purposes.
Good luck--- HHH, MD
Local clinics and CDC indicate a Syphilis increase in gay males and I assume I was equally at risk for this as well from receiving b-job (insertive fellatio) (not reciprocating).
The info I found shows that Syphilis is also a bacterial infection so I assume your answer will be yes.
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