Abstract Pharmaceuticals and recently also illicit drugs have been recognised as emerging environmental contaminants due to their potential environmental impact: frequent occurrence, persistence and risk to aquatic life and humans. This manuscript is part one of the two-part study aiming to provide a better understanding and application of environmental data not only for environmental aims but also to meet forensic objectives. An attempt to use wastewater data is made in order to verify patterns of the usage of drugs (in particular illicit) in local communities. The average usage of cocaine in South Wales was estimated at 0.9 g day-1 1000 people-1, which equals 1 tonne of this drug used or disposed of to sewage annually in Wales. The calculated usage of amphetamine denoted 2.5 g day-1 1000 people-1 and is suspected to be an overestimate. Because no analysis of enantiomers of amphetamine was undertaken, no distinction between amphetamine's legal and illicit usage could be made.