In recent decades, there has been an enormous increase in drug usage in human and veterinary medicine around the world. Drugs can enter the environment in parent form or as metabolic by-products; the primary source of contamination of the aquatic ecosystem is wastewater from hospitals and households. The purification technologies used in wastewater treatment plants are in most cases able to eliminate the given drugs only partially, for which reason surface and ground water can easily be contaminated with these substances. This paper describes the effects of pharmacologically active substances, the most frequent contaminants of surface waters in the Czech Republic, on fish and other aquatic organisms.