Controlled substances refer to either natural or manufactured chemicals that act as stimulates or sedatives to the brain. Not all controlled substances are illegitimate—many have valid medical uses but are considered to have a higher than average potential to cause addiction or dangerous abuse if dispensed without medical supervision. Controlled substances range from common street drugs to prescription medications having a limited potential for abuse.

The U.S. Drug Enforcement Agency ranks controlled substances in classifications called schedules, numbered from I to V. The higher the schedule number, the less hazard posed by the drug to users. Federal law enforcement generally considers illegal sales or abuse of substances in schedule I and schedule II to be the object of the most vigorous enforcement.

Prescriptions for controlled substances are generally under both federal and state regulations. These rules regulate how a prescription is written and the accepted ways it can be conveyed from the doctor to the pharmacist. Regulations also specify how many times a prescription for a controlled substance can be refilled. For controlled substances, it is typical that refills for a prescription can be issued for only up to 6 months from the original date of the prescription. Under most circumstances, prescriptions for controlled substances classified as Schedule II cannot be refilled at all.

Drugs classified as Schedule I controlled substances include street drugs such as heroin, LSD, ecstasy, peyote, psilocybin, methaqualone (also known as Quaalude) and tetrahydrocannabinols.

Drugs classified as Schedule II controlled substances include unprocessed opium and extracts, morphine, codeine, cocaine and crack, amphetamine, methadone, methamphetamine and phencyclidine (also known as PCP or Angel Dust).

Drugs classified as Schedule III controlled substances include dilute or less concentrated formulas of codeine and anabolic steroids.

Drugs classified as Schedule III controlled substances include diazepam (also known as Valium,) chloridazepoxide (also known as Librium,) pentazocine pain killers and phentermine weight-loss drugs.

Drugs classified as Schedule IV controlled substances include very dilute, non-concentrated forms of codeine, most commonly prescribed anti-depressants and some stimulants used by athletes to enhance performance (known as “doping.”)