A drug can be defined simply as any chemical (or substance)
which when taken , leads to changes in working of normal body
system.
A drug can be natural like THC found in marijuana or
industrially produced like paracetamol.
Drugs may target any system of the body :
nervous/circulatory/excretory etc.
Drugs have a range of effect, depending on the quantity of
intake. There is a optimal value which has positive effects on
body, and beyond a dosage range, effects are negative.
For eg. a medicine when taken in prescribed quantity can
provide relief from an illness. But, if the same medicine (drug) is
taken in excess, it may lead to deleterious effects.
So , in a broader view everything is a drug, from the water we
drink to food we eat to oxygen we inhale. Excess water can lead to
water intoxication and excess oxygen leads to hypoxia (toxic).