Illegal drugs are those that are not prescribed by a doctor or bought at a drugstore. They are sometimes also called "street drugs."

Some examples of illegal drugs include heroin (an opioid); cocaine (a stimulant); LSD, mescaline, PCP, methamphetamine, and psilocybin (hallucinogens); and gamma-hydroxybutyrate (GHB). GHB is legal in Europe for anesthetic uses and in the United States for government-approved clinical trials for treating narcolepsy. But it has been otherwise illegal in the United States since 1990.

The effects of illegal drugs can be unpredictable because:

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