Drug cultures are examples of countercultures, primarily defined by recreational drug use.
Drug cultures may be seen as groups of people loosely united by a common understanding of the meaning and value (good or otherwise) of the incorporation into life of the drug in question. Such unity can take many forms, from friends who take the drug together, possibly obeying certain rules of etiquette, to full-scale political movements for the reform of drug laws. The sum of these parts can be considered an individual drug's "culture".
It should be noted that there are multiple drug cultures based on the use of different drugs - the culture surrounding cannabis, for example, is very different from that of heroin, due to the different sort of experiences and problems the drugs cause.