You know, PrEP, Pre-exposure prophylaxis (and PEP for post-exposure), is not a drug itself but is a mix of approved medications (in theory, any approved mix?) I think you may be thinking of what happens when they (trial?) doxycycline, the antibiotic, in STI prophylaxis? Proof is a v strong word, and I haven’t a clue either way because I haven’t read enough. However, in Ireland, we use tenofovir/emtricitabine disoproxil (anti-vitals) in our PrEP neither of which combat the bacteria responsible for Syphilis. You’re English though, aren’t you? I don’t know what goes on over there, or what they might be giving you on top of your PrEP…
Link regarding doxycycline as prophylaxis for Syphilis (not in Ireland) >>
here and
here
There is the friendly neighbourhood GOSHH, you could ask them >>
here
All very interesting, isn’t it.