I'll keep it short. Exposure: condom protected sex (vaginal and oral) with a CSW in Europe. There was also kissing, not deep kissing really just on the lips but for a good while. The condom was checked after the brief sex and was intact. I understand the low risk for viral infections from condom protected sex. But I got a sore on my inner lip 5-6 days after the incident. I get canker sores every now and then and it look and feels like one. It hurts when I touch it with my tongue or if it gets into contact with certain food. This got me thinking of syphilis. I know that syphilis is rare. Plus i checked that accoring to CDC symptoms appear 10 - 90 days after exposure (avg 21 days) and that the chancre is painless. But could it still be syphilis? According to images it looks a lot like canker sores. And symptoms/timeline, is it the same for oral syphilis? Or would the timeline and the fact that there is pain rule out syphilis or at least make it unlikely? Should I test and if so, when?

You have pretty well evaluated the risks:  Low chance any particular female sex worker has syphilis, low risk from kissing (especially without deep kissing), timing of your oral sore too soon for syphilis from that event, that syphilitic chancres usually are not painful, and finally that the similarity of your oral sore to past aphthous stomatitis (canker sores) strongly suggests that's what it is -- another canker sore. I would put the odds you have syphilis at well under one chance in several million.

Should you be tested for syphilis? From a medical/risk perspective, I see no need at all. But from a personal standpoint, you might still want to be tested. Most people sufficiently worried to come to this or similar forums are likely to stay worried until tested with negative results -- that is, professional opinion may not be sufficient, no matter how expert it may be. (We don't take it personally!) So if you're going to lie awake worried about syphilis despite my assessment, by all means get tested. In that case, you can have a conclusive syphilis EIA test (i.e. for IgG/IgM antibody) 6 weeks after the event.

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

HHH, MD

Still a few follow ups:

1. If i get anxious and want to rule out syphilis for the mentioned mouth sore, how soon after the sore appeared would a test be positive? Or is it always 6 weeks post exposure.

2. Apart from rarity, is the timing the strongest evidence against syphilis? If most credible sources state 10-90 days, I guess 5-6 days should be very rare if possible, otherwise I guess it would have made sense to state for example 7-90 days. 

3. The sore has started healing after a week, can still feel it though. It had a white middle part like canker sores do. Do syphilis sores have this at all or is this further evidence against it.

Despite my questions, I am not overly anxious and the topic can be closed after your final comments, thanks again.

1. Testing can be positive as soon as soon as a week after onset, but usually longer. A negative result at 4 weeks would be highly reassuring, although not quite conclusive.

2. I gave several reasons why syphilis is unlikely. All are equally important. (90 days is the outside range of the testing window. Symptom onset of primary syphilis almost never goes anywhere near that long.)

3. Sounds like a typical canker sore to me. However, syphiltic chancres are too variable to conclude anything definite. But as I said, other factors also make syphilis almost impossible in this situation.

That completes the two follow-up comments and replies included with each question and so ends this thread. I hope the discussion has been helfpul.