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IntroductionPre-exposure prophylaxis (PrEP) is an effective prevention measure against HIV. Risk compensation may partially counteract the public health effect of PrEP if this might increase STI incidence. We studied changes in sexual behaviour among men who have sex with men (MSM) and transgender women (TGW) who started daily PrEP In the Amsterdam PrEP (AMPrEP) demonstration project over the first 6 months.MethodsParticipants completed a questionnaire at baseline and at the 6 mo visit, about sexual behaviour in the preceding 3 mo. At baseline information about demographics and drug use was collected. Sexual behaviour questions concerned frequency of sex, condom use and sexual position (insertive/receptive) by partner type (casual/steady). Reported sexual behaviour at 6 mo was compared to behaviour at baseline using signed rank tests. Logistic regression was used to identify predictors for an increase in receptive condomless anal sex acts (rCAS) with casual partners.ResultsIn 2015–2016, 273 participants started daily PrEP. From this analysis 49 participants were excluded because they switched to intermittent PrEP (n=23), were not due for their 6 months visit yet (n=17), or because of missing data (n=9). The total no. of sex partners (median=15) and no. of anal sex acts (median=24) remained unchanged (p=0.2, p=0.4, respectively). The no. of casual partners increased (median from 14 to 15, p=0.03). The total no. of rCAS increased from a median of 3 to a median of 8 (p<0.001). The same trend was seen for rCAS with casual partners (median from 2 to 5, p<0.001). In multivariable analysis, age ≥35y (p=0.058) and chemsex (GHB/GBL, mephedrone, crystallised methamphetamine) (p=0.003) were associated with an increase in rCAS with casual partners.ConclusionDuring the first 6 mo of daily PrEP use, we observed an increase in sexual risk behaviour among MSM and TGW. PrEP users aged ≥35y and those engaging in chemsex were more likely to report an increase in high risk sexual behaviour. Whether risk compensation leads to an increase in STI incidence needs to be closely monitored.