Meaning of “drug” - Learner’s Dictionary

drug

noun [ C ] uk us /drʌɡ/
ILLEGAL SUBSTANCE

B2 an illegal substance that people take to make them feel happy:

[ usually plural ] He started taking/using drugs such as heroin and cocaine.
Greg is on drugs (= he uses drugs regularly).
a drug dealer
MEDICINE

B2 a chemical substance used as a medicine:

Scientists are developing a new drug to treat cancer.
→ See also hard drugs

(Definition of “drug noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)