The use of more than one drug or type of drug by an individual, often at the same time or sequentially, and usually with the intention of enhancing, potentiating, or counteracting the effects of another drug. The term is also used more loosely, to include the unconnected use of two or more drugs by the same person. It carries the connotation of illicit use, though alcohol, nicotine, and caffeine are the substances most frequently used in combination with others in industrialized societies.
Multiple drug use disorder (ICD-10 F19) is one of the ''Mental and behavioural disorders due to psychoactive substance use" in ICD-I0, diagnosed only when two or more substances are known to be involved and it is impossible to assess which substance is contributing most to the disorder. The category is also used when the exact identity of some or even all of the substances being used is uncertain or unknown, since many multiple drug users often do not know themselves what they are taking.