Pharmaceuticals have recently emerged as novel pollutants of potential concern in the aquatic environment where they are commonly introduced as complex mixtures via municipal effluent. In the present experiment, the freshwater cnidarian Hydra attenuata was exposed to a mixture of 11 pharmaceuticals (ibuprofen, naproxen, gemfibrozil, bezafibrate, carbamazepine, sulfapyridine, oxytetracycline, novobiocin, trimethoprim, sulfamethoxazole and caffeine) up to 10000 times (×) the concentration found in municipal effluent. Hydra regeneration and teratogenicity was measured, having an IC50 of 781× and was found to be non teratogenic with an A/D value of ∼1. Toxicity was investigated using both lethal (based on morphology) and sub-lethal (based on morphology, feeding behaviour, hydranth number and attachment) endpoints. The pharmaceutical mixture incurred a significant decrease in morphology at 0.1, 10 and 100× but a significant increase at 1000×. All parameters were significantly reduced at 10000×. An EC50 of 425× and 65× based on morphology and feeding respectively and a toxicity threshold (TT) of 3.2× were calculated. When compared to the toxicity of each pharmaceutical exposed individually as previously reported [Quinn B, Gagné F, Blaise C. An investigation into the acute and chronic toxicity of eleven pharmaceuticals found in wastewater effluent on the cnidarian, H. attenuata. Sci Total Environ 2008a; 389: 306–314], the compounds in the mixture were present at concentrations 2 to 3 orders of magnitude lower for the equivalent toxicity (EC50 and TT). These results indicate that pharmaceuticals act additively in a mixture, having sub-lethal effects at environmentally relevant (µg/L–ng/L) concentrations and that their combined concentrations could potentially prove significantly ecotoxic to Hydra and possibly to other aquatic taxa. [Copyright &y& Elsevier]