Illegal drugs are those that are not prescribed by a doctor or bought at a drugstore. They are sometimes called "street drugs." Some examples include heroin (also called smack, junk, or dope), cocaine (coke or crack), PCP, ecstasy (MDMA), Rohypnol (roofies), and meth (crank or speed).

The effects of illegal drugs can be hard to predict because:

• People respond differently to the same drugs.
• The strength or potency of the drug may be uncertain.
• They may contain unknown substances or dangerous chemicals, such as fentanyl. There is no quality control for street drugs like that required for prescription drugs.
• They may interact with other drugs or alcohol in harmful or unpredictable ways.