STI prophylactic antibiotics are a single dose of a specific antibiotic that you take after sex to prevent sexually transmitted infections. Taking it can reduce your risk of getting an STI such as syphilis or chlamydia by up to 80% or more. For gonorrhoea, STI prevention is less effective, but still partially effective.
Studies have shown convincing results for its effectiveness in cis-men who have sex together and in trans women. The German STI Society has summarised the studies in a position paper. The final German recommendations from these studies are still being discussed by German medical societies, particularly due to the potential development of resistance, disruption of the microbiome and possible lax use of antibiotics. However, given the compelling evidence, many countries, including the US and Canada, have already adapted local guidelines and regularly use antibiotics for STI prophylaxis. antibiotic recommendations for STI prophylaxis are also part of the European HIV PREP guideline.
Remember that antibiotic STI prophylaxis does not protect against HIV! The antibiotics commonly used for STI prophylaxis are very safe antibiotics and have been on the market since the 1960s. This class of antibiotics is still widely used in human and veterinary medicine.
Overall, there is very good experience of long-term use in various areas of treatment with low rates of side effects (e.g. acne, rosacea) or prophylaxis (malaria, leptospirosis, Lymes disease).
Currently, antibiotic STI prophylaxis is still an off-label use. This means that you won't find STI prophylaxis mentioned on the package leaflet, but the possible side effects are no different from other uses.