Drugs essentially are poisons. The degree they are taken determines the effect. A larger amount acts as a poison and can kill one. This is true of any drug. Each requires a different amount.
Caffeine is a drug, so coffee is an example.
One hundred cups of coffee would probably kill a person. Ten cups would probably put him to sleep. Two or three cups stimulates. This is a very common drug. It is not very harmful as it takes so much of it to have an effect.