Addiction: A chronic, relapsing disease, characterized by compulsive drug seeking and use and by neurochemical and molecular changes in the brain.

Agonist: A chemical compound that mimics the action of a natural neurotransmitter to produce a biological response.

Analog: A chemical compound that is similar to another drug in its effects but differs slightly in its chemical structure.

Antagonist: A drug that counteracts or blocks the effects of another drug.

Buprenorphine: A mixed opiate agonist/antagonist medication for the treatment of heroin addiction.

Craving: A powerful, often uncontrollable desire for drugs.

Detoxification: A process of allowing the body to rid itself of a drug while managing the symptoms of withdrawal; often the first step in a drug treatment program.

Fentanyl: A medically useful opioid analog that is 50 times more potent than heroin.

Meperidine: A medically approved opioid available under various brand names (e.g., Demerol).

Methadone: A long-acting synthetic medication shown to be effective in treating heroin addiction.

Physical dependence: An adaptive physiological state that occurs with regular drug use and results in a withdrawal syndrome when drug use is stopped; usually occurs with tolerance.

Rush: A surge of euphoric pleasure that rapidly follows administration of a drug.

Tolerance: A condition in which higher doses of a drug are required to produce the same effect as during initial use; often leads to physical dependence.

Withdrawal: A variety of symptoms that occur after use of an addictive drug is reduced or stopped.