Hello , Welcome to Healthcare Magic forum. Flucloxacillin is an antibiotic and Diphenhydramine is an antihistamine medication. There are no known drug interactions between the two .Thus they can be taken together as prescribed. However each has its own side effects like diarrhoea, allergic reaction with Flucloxacillin and drowsiness, headaches, stomach upset (diarrhoea, nausea etc,)with diphenhydramine. Not everyone will suffer from all side effects and usually they are mild and self resolving once treatment is stopped. Morever taking the medicines together won t increase or decrease the incidence of individual side effects. So kindly take these medications only as prescribed . Hope I have answered your query. Regards