Doper (noun)

1. A person who takes illegal drugs.
2. A person who takes performance enhancing drugs, especially in sports.

Origin:

Mid 20th century: from dope (sense 2 of the noun).

Examples:

1. The authorities arrested several dope dealers and users in the raid.
2. Many professional athletes have been banned for using performance enhancing drugs, known as dopers.
3. The athlete failed the drug test and was stripped of his title as a doper.
4. The doper tried to hide his habit, but eventually was caught.
5. His friends tried to intervene and stop him from becoming a doper, but to no avail.