Definitions of drug

(also: narcotic) an illegal substance taken for its physical or mental effects

I've never taken drugs.

a medicine taken to treat an illness or a disease

The drug will begin trials shortly and may be available on prescription within 18 months.

A more formal word for drugs is narcotics, and the word narc is a slang term used to describe a police officer who specializes in drugs cases. But the word narc was once used to refer to a police informant, someone who gives information about criminals to the police, and could also be used to refer to the police in general. In that sense of the term, the word derived from the Romany word nak, which means nose. In other words, a police officer or a police informant is someone who sticks their nose into other people's business.