Hello doctors. I consider myself a heterosexual male and am in a committed relationship with a female. I recently had an encounter with a male I met online thru an app called sniffies. He also is married to a woman and from what he said he doesn’t have any reason to believe he has any stds. I don’t know if I can trust that but we did speak afterwards and he stated that he is clean. I gave him a protected blowjob using a condom on 3/16/26. It must have lasted no more than 2 minutes. After that I stopped and we just continued to masturbate. The only contact that was made was with my mouth on his penis and he wore the condom for the whole duration. I immediately panicked afterwards and that same night I started a ten day course daily of amoxicillin Clav 875/125 mg. That same night I also took doxycycline 200mg but realized that it had expired 5 months previously. I was able to get ahold of another doxycycline 200mg dose and took that 46 hours post the protected blowjob I gave. I went to test for syphilis on 4/6 which was 3 weeks post the event. My results came back today as negative. I do have the following questions regarding the incident that transpired and the antibiotics I took. 

I have to specify that the amoxicillin Clav 875/125 mg I took was one single capsule daily for ten days. I tested for syphilis 10 days after I had stopped the amoxicillin

Welcome to the Forum.  Thanks for your questions.  I'll be glad to comment.  To cut to the chase, the likelihood that you were even exposed to syphilis is close to zero and, had you been, both the doxycycline and the amoxicillin/clavulatate you took would have prevented infection had you been exposed.  While your test results are technically not conclusive at this time, they are further strong evidence that you did not get syphilis from the encounter you describe.  Epidemiologically your partner was low risk, the presence of the condom for your encounter would have substantially prevented syphilis and as I said, the drugs you took would have prevented infection had you been exposed..  To address your specific questions:---

Thank you much for that detailed response, Dr. Hook. I did have a couple of follow questions. As I mentioned, the initial dose of doxycycline that I took that same night after the encounter was expired by about 5 months. Is it still effective despite it being expired 5 months? I did take the second dose of doxycycline 200 mg 2 days after the event and that dose wasn’t expired. I’m hoping that just added more protection if had I been exposed as you mentioned. 

You can only kill the bacteria that caused Syphilis once. Despite being past your expiration date, the first dose of doxycycline you took would be effective. Subsequently the doses of doxycycline and the amoxicillin you took would also kill the bacteria.---

Thank you doctor. I did have one last question. It’s more of a hypothetical. In the event that the doxy didn’t work in preventing exposure of syphilis had I been exposed, you mentioned that the amoxicillin would not have worked either as I was taking one tablet of amoxicillin 875 a day for 10 days instead of taking twice daily. My question is if that were the case, would the amoxicillin I took “mask” any -“antibodies or positive results including the test I took 10 days after I stopped the amoxicillin had I been exposed to syphilis? In your honest opinion, given the scenario of me giving a blowjob with a condom on the penis, did I even need to worry about testing in the first place? I believe this would be your last follow up response. Thank you again doctor. 

I wanted to clarify that the syphilis test I took was 21 days after the encounter and 10 days after I stopped taking the amoxicillin. 

Final responses.  Syphilis is easier to prevent than to cure.  There is no evidence that acquired syphilis.  In fact, your blood test shows that you have not.  That said, in the most unlikely situation that two doses of doxycycline would not have worked (hypothetically) the amoxicillin would have effectively prevented infection.  ---