STI rates in PrEP users are high, but PrEP may not cause STI rates to rise
A recent study presented by Professor Matthew Golden of King County's STI and HIV program shows that PrEP users have STI diagnosis rates 20 times higher than that among the general population of HIV-negative gay men. Additionally, STI diagnoses increased in the period from one year before subjects sought PrEP to the date they started it, and the percentage of men who reported never using condoms during anal sex increased slightly while they were using PrEP (though the proportion was never more than 10% of all PrEP users).
However, there was only ambiguous evidence that people on PrEP contracted more STIs while using the medication. While chlamydia diagnoses did increase in the nine-month period after starting PrEP, syphilis diagnoses decreased over the same period and gonorrhoea cases remained stable. Urethral gonorrhoea increased during PrEP use, but only in a small number of people.
Other studies have reached various conclusions. The Kaiser Permanente PrEP program in northern California found that STI rates increased after subjects started PrEP, but the PROUD study did not reach the same conclusion.
It is difficult to show that PrEP stands in a causal relationship to STIs for two reasons: STI diagnoses had been rising among gay men well before PrEP came along, and PrEP involves regular testing for HIV and other STIs. The study may simply show that people who are at higher risk for STIs also tend to seek PrEP, and that PrEP testing procedures are facilitating diagnosis of asymptomatic STIs that would otherwise go undetected.