Although there are different methods that are used to classify drugs, the most common scheme groups drugs by their pharmacological similarity. However, [Page 331]this scheme does not work well for "club drugs," which are discussed as a separate classification. This entry describes, for each drug classification, the common drugs contained in the classification, some common street names, major effects, signs of intoxication, signs of overdose, tolerance, withdrawal, and acute and chronic effects. It should be noted that the term drugs includes both legal and illegal mood-altering substances.