Polydrug use refers to the use of more than one drug. Users often have a primary drug of choice (for example alcohol, marijuana, speed or heroin) but will use one or more drugs to top up, come down or as a substitute. Combining drugs can increase or alter their effects, often in unpredictable ways.

Despite the risks, polydrug use is quite common. For example, tranquillisers or marijuana may be used to help a person come down from speed or an LSD trip. Using two or more drugs simultaneously or within a relatively short timeframe can be hazardous. There is a real risk of overdose if, for example, two drugs that are both central nervous system depressants are present in the body at the same time. Taking heroin, minor tranquillisers, methadone and alcohol (in any combination) can be fatal.