So I had a situation with a sex worker. I had unprotected oral sex with her (fellatio) 40 days ago.  I was very stupid and anxiety is killing me. I went to an urgent care 5 days after exposure because I felt burning when I was peeing. The doc prescribed bactrim DS and I took it for 10 days (2 pills per day) because she thought it could have been UTI. Anyways, pee culture clean, gonorreah and clamhydia clean after two weeks of exposure. HIV RNA after 10 days negative and HIV 4th gen after 4 weeks also negative.  Siphilis RPR test after 2 and 4 weeks also negative. I’m taking test a test tomorrow for syphilis and hiv to make it 6 weeks and conclusive. Would bactrim delay syphilis symptoms such as chancre (to this day I have none) or I am good taking the test at the 41st day (~6 week)?

I’m over observing my genitals and I just want to stop doing this so I can get closure. Thank you for your answer and work!

Some questions can be accurately answered based on only the title the user chooses. Sulfamethoxa/trimethoprim (Bactrim) has zero activity against syphilis. If taken at the time of exposure, it will not prevent it; and if taken for attempted cure, it doesn't work.

Now I have read the question itself. In reply to your main question, the exposure was extremely low risk for syphilis and your negative blood test at 4 weeks is reassuring:  not quite conclusive until 6 weeks, but for final reassurance, go ahead with your planned repeat test at 41 days (just as good as 42 days); it will be negative. The SMX/TMP (Bactrim) cannot change or delay a positive test result.

Your urinary discomfort 5 days after the event probably was no infection at all from the oral sex event. Oral sex doesn't cause UTI; and although it can transmit gonorrhea or nongonococcal urethritis (NGU) due to mouth bacteria, burning alone usually isn't due to any infection; discharge of mucus or pus from the penis is the main symptom. SMX/TMP doesn't treat any of these problems. The initial burning may have been your self-described "over observing" your genitals. Your anxieties over the oral sex event likely made you more aware of a minor symptoms or even normal body sensations that otherwise you would ignore or not even notice. (Your doctor might need education about oral sex and its risks. If any infection at all was suspected, SMX/TMP was the wrong drug for all possiblities. Did you inform her about the sexual exposure?) Anyway, it's good your gonorrhea and chlamydia tests were negative. If the "burning" is gone and you have another negative syphilis blood test, you can forget the whole episode.

Receiving oral sex is zero risk for HIV. The virus has never been scientifically documented to be transmitted oral to penis. And the negative HIV test results are conclusive, proving you don't have HIV; you don't need another test at 6 weeks or any other time.

I hope these comments are helpful. Let me know if anything isn't clear.

HHH, MD

About the doctor who treated me, I did explain her what happened and my fears and I was expecting doxy prep or something like that, but she decided to go with bactrim.

I will stop “over observing” myself because anything slightly red makes me overthink. As I said, no chancre for 40 days or nothing close to it, so I think I should be fine. Thank you again doctor.  

Doxy-PEP would not have been appropriate in this situation. Bactrim might have been a good option if you actually had a UTI but I doubt you did.

Thanks for the thanks!

I just got my results and the result was what you predicted. They are negative. I will move on with my life and forget (and learn) from this situation. Again thank you so much for all of your help and have a great day!