Pharmaceuticals are accumulating in environments globally. This includes trenbolone, a growth promoter that enters waterways in agricultural run-off. However, whether and how drugs like trenbolone impact complex behaviours in wildlife remain largely unknown. We exposed male guppies (Poecilia reticulata) to trenbolone and compared the response of exposed and unexposed males to sequentially presented large and small females. Due to a positive size-fecundity relationship, larger females are generally expected to be preferred by males. While we found no evidence that the size of a previously encountered female affected the amount of mating behaviour performed by males during the second presentation, males from both exposure treatments conducted more frequent courting events towards larger females during both presentations, suggesting an absolute preference for greater female size. Further, across both presentations, trenbolone exposure caused a shift in male mating strategy towards increased sneaking behaviour, although male sequential investment into mating effort was not impacted. Our findings contribute to a growing understanding of impacts of pharmaceuticals on wildlife behaviour.