A diverse cocktail of pharmaceuticals is spreading via water treatment plants’ effluent to surface water with known and unknown consequences of individual organisms and the ecosystem. In prior research on what consequences the pharmaceutical discharge can have, the antihistamine diphenhydramine has been found in surface waters and has been linked to alteration in organisms’ behaviours. In this study, Dytiscidae larvae were filmed before and after being exposed to diphenhydramin with a concentration of 1 µg/l. The exposure was done using four different treatments: no diphenhydramine, diphenhydramine in water, in their food or in both water and food. This was done to observe whether the behaviour, specifically activity and boldness, of Dytiscidae larvae would change with the exposure of the antihistamine and therefore potentially be a problem in aquatic ecosystems exposed to effluent. The experiment showed a significant difference (P = 0.015) between larvae in the control group and larvae only exposed to diphenhydramine through water, where exposed larvae were less active than the control group. Also, a trend pointed to a potential difference in exploration between the same groups, where the group exposed to diphenhydramine explored more than the control group. No behavioural change was found when larvae was exposed via food or food and water. The changed behaviour found here could influence fitness of the larvae and potentially also other parts of the ecosystem through cascading effects. Further research is required to confirm the dangers and consequences of diphenhydramine in surface waters, but my results suggest that it can alter the organism’s behaviour in aquatic ecosystems.