Background PrEP with FTC/TDF has shown great efficacy in preventing new HIV infections but issues remain (low adherence, high costs, toxicity and resistance development). No data are available about the impact of circulating Resistance-Associated Mutations (RAMs) on its efficacy. Objectives describe the prevalence of FTC and/or TDF-related RAMs in Italian HIV-infected population and their potential impact on PrEP efficacy. Study design ARCA is a national database that collects data about RAMs and epidemiological correlates from sites throughout Italy; it was queried about the prevalence of these RAMs in the last decade. PrEP efficacy was adjusted for a dynamic score based on RAMs prevalence. Absolute and relative risk increases (ARI and RRI) and number needed to harm (NNH) were calculated after this score. Results the query retrieved 3579 HAART-naïve and 5781 experienced subjects. Resistance to TDF is low and more common among naïve MSM in the area of Milan (where it topped to 14.3%), without other significant differences. If good adherence is not attained, RRI for receptive anal sex increases by 16% (in naïve) and 93.4% (in experienced MSM). NNH is largely above 10000 except for having receptive anal sex with a HAART-experienced MSM on a failing treatment (970). Conclusions according to this model, PrEP may be introduced in Italy without general concerns, but efficacy may be partly reduced in young MSM having sex in Rome and Milan.