According to clinical studies compiled by the German STI Society, antibiotics used for STI prophylaxis showed reduction rates for:
Syphilis by 73% to 87%
Chlamydia by 70% to 89%
Gonorrhea by 17% to 57%
On average, across the mentioned STIs, by 47-84%
These numbers can change over time and, especially in case of gonorrhea, depend if the antibiotics still work in your region, for example in Germany statistics show less then 10% efficacy for gonorrhea in antibiotics commonly used for bacterial STI-Prevention. For more information, see our website.