Twelve pharmaceuticals comprising anti-inflammatory agents (ibuprofen, naproxen), lipid regulators (bezafibrate, gemfibrozil), an anti-convulsant (carbamazepine), antibiotics (sulfamethoxazole, sulfapyridine, oxytetracycline, novobiocin, trimethoprim), a stimulant (caffeine) and a nicotine metabolite (cotinine) were identified in the final effluent of a major wastewater treatment plant (WWTP) based in the Montreal area (Québec, Canada) discharging to the Saint-Lawrence River. Their measured concentrations ranged from 33 (carbamazepine) to 22187 ng/L (caffeine) in the Montreal WWTP effluent and from 58 (cotinine) to 85000 (ibuprofen) ng/L based on maximum concentrations found in other municipal effluents as reported elsewhere. In general, elimination of these drugs by wastewater treatment is purported to be fairly efficient for ibuprofen, naproxen, bezafibrate, gemfibrozil and caffeine. In contrast, carbamazepine, sulfamethoxazole, sulfapyridine and oxytetracycline appear more resistant to treatment whereas removal efficiency for novobiocin, trimethoprim and cotinine is either unclear or unknown. Despite the degree to which they are eliminated in WWTps, their presence in urban effluents discharging to surface waters is common. To estimate their potential risk to aquatic biota, we undertook acute/chronic toxicity testing with a suite of small-scale bioassays representing four taxonomic groups (bacteria, algae, invertebrates and fish) and complemented our results with those published in the literature. When all acute and chronic toxicity tests were combined, their toxicity responses spanned over five orders of magnitude. Barring caffeine and cotinine, all others are clearly recognized as toxic based on the EU-Directive 93/67/EEC classification, but their concentrations measured in WTTPs are too low to cause acute or chronic effects on an individual basis. Sulfamethoxazole, naproxen and ibuprofen, however, were found to produce toxic effects at concentrations that are <= one order of magnitude to those found in municipal effluents. In general, bioassay responses displayed a wide range of sensitivity that justifies continuing use of representative species within test batteries to properly appraise the toxic potential of pharmaceuticals. We also showed that exposure of Hydra attenuata to caffeine for 48 h induced xanthine oxidoreductase activity which may in turn lead to production of oxidative stress at concentrations close to those present in urban effluents. Future studies on the ecotoxicity of pharmaceuticals should therefore also seek to include appropriate biomarkers to assess possible adverse long-term effects to biota resulting from the biotransformation of drugs.