Azithromycin is on Medirabbit’s list of rabbit-safe antibiotics. Safe Antibiotics (medirabbit.com)

For rabbit syphilis, injectable penicillin is more commonly used though. It’s only given as injections so it doesn’t have to go via the GI tract to work.

Both penicillin (injected) and azithromycin are safe treatments for syphilis.

In my understanding, syphilis itself isn’t going to cause sneezing, so I would explore whether your rabbit’s suppressed immune system (as a result of having syphilis) may be facilitating the emergence of a upper respiratory infection. Anecdotally, my rabbit Wick had to be on antibiotics for 5mo to get rid of a URI, and we later found out it had syphilis the entire time, so it may have been him having syphilis that made it so unusually hard to fight the infection…. but that’s something we can’t test obviously. Of note, Wick had the atypical form of syphilis, which only presents with lesions on the face and none of the genitals.

Wick’s experience with azithromycin did show some gastric distress, though it lessened overtime and I discovered (unsurprisingly) that as long as I gave him banana after a dose, he would feel comfortable eating again, haha.