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To improve the risk assessment of pharmaceuticals, it is helpful to know how rapidly they are removed from river water. Direct photolysis by sunlight could be an important process, but so far few studies have attempted to compare modeled with actual losses in a river. Therefore, we quantified natural attenuation by monitoring 56 pharmaceuticals and personal care products over 2 full years in a 2.6 km stretch of an urban river. In addition, to screen photoproducts, we used the Microtox test with Vibrio fischeri to evaluate changes in the toxicity of two photolabile pharmaceuticals, ketoprofen and diclofenac, under sunlight. During transport along the river stretch, ketoprofen and the photolabile pharmaceutical furosemide were attenuated by median values of 77% and 39%. The observed attenuation showed good agreement with photochemical attenuation estimated by an existing method at each sampling, suggesting that the method appeared to be effective for estimating the direct photolysis of the pharmaceuticals during river transport. The toxicity of diclofenac decreased under sunlight, while that of ketoprofen increased immediately after exposure (around 12 times in EC20) and remained high, indicating the existence of toxic and photostable photoproducts of ketoprofen. Therefore, ecological risks of photolabile pharmaceuticals may increase during river transport in some cases, indicating the necessity to incorporate their photoproducts into the estimation method.

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