It would be good to know more about your partner's syphilis, specifically the stage, duration, and whether he had been treated. Syphilis can be sexually transmitted only during the first year (and mostly during the first 3-6 months) after infection. And if someone has had any treatment at all, it cannot be transmitted. And oral sex is low risk.
If you were exposed, the azithromycin likely would have prevented active infection; and doxycycline certainly will prevent it. The preventive treatment of choice is penicillin by injection; unless you are allergic to penicillin, that would have been preferable. But the doxy is fine as long as you take all of it as prescribed. Therefore, the rest of your questions are more or less irrelevant: even if exposed, you can be totally confident you won't have any problem. To your specific questions:
1) Very low risk for the exposure you describe, especially since there were no visible lesions. 2) Probably the azithromycin took care of it, if you were exposed. There are azithromycin-resistant strains of syphilis, however. (If you happen to be in San Francisco, there is a strong chance of azithromycin resistance.) 3) Syphilis cannot be transmit to another person until at least 3 weeks after infection, even without antibiotics. Either doxycycline or azithromycin will effectively prevent transmission within a few hours of the first dose. 4) Syphilis is most readily transmitted by vaginal or anal intercourse. Oral can do it, but less frequently. There are no data on whether transmission is more efficient in one direction or the other.
Bottom line: You're home free; don't worry about syphilis. (Presumably you are being tested for HIV. For practical purposes, every case of syphilis occurs in someone at high risk for HIV infection.)
Good luck-- HHH, MD
Syphillis is a bacteria infection that can be transmitted orally. However the Azithromycin you were put on for other reasons would prevent the growth of t. pallidum, (the cause of syphillis.) The doxycycline you are currently on is also a good indicator that you prevented the chance of actually aquiring it.
Thank you for your response. I was tested for HIV, but the guy in question tested negative, so I am not worried about it. So, the second antibiotic I took would have started working within a day or so of taking the dose needed?
Thanks again for your responses...you provide a service that helps so many people. Have a good day.
As I said, "Either doxycycline or azithromycin will effectively prevent transmission within a few hours of the first dose."
Thanks for the thanks-- HHH, MD