Drug addiction, or dependence, is the compulsive use of a substance, despite its negative or dangerous effects.

However, a physical dependence on a substance (needing the drug to function) is not always part of the definition of addiction. Some drugs (for example, certain blood pressure medications) do not cause addiction but they can cause physical dependence. Other drugs cause addiction without leading to physical dependence. Cocaine is an example.

Tolerance to a drug (needing a higher dose to attain the same effect) is usually part of addiction.

See also:

• Drug abuse
• Drug abuse first aid
• Cocaine intoxication
• Cocaine withdrawal
• Opioid intoxication
• Opioid withdrawal

Drug abuse can lead to drug dependence or addiction. People who use drugs for pain relief may become dependent, although this is rare in those who don't have a history of addiction.

The exact cause of drug abuse and dependence is not known. However, a person's genes, the action of the drug, peer pressure, emotional distress, anxiety, depression, and environmental stress all can be factors.

Peer pressure can lead to drug use or abuse, but at least half of those who become addicted have depression, attention deficit disorder, post-traumatic stress disorder, or another mental health problem.

Children who grow up in an environment of illicit drug use may first see their parents using drugs. This may put them at a higher risk for developing an addiction later in life for both environmental and genetic reasons.

Commonly abused substances include:

• Opiates and narcotics are powerful painkillers that cause drowsiness (sedation) and feelings of euphoria. These include heroin, opium, codeine, meperidine (Demerol), hydromorphone (Dilaudid), and oxycodone (Oxycontin).
• Central nervous system stimulants include amphetamines, cocaine, dextroamphetamine, methamphetamine, and methylphenidate (Ritalin). Caffeine and nicotine are the most commonly used stimulants. These drugs have a stimulating effect, and people can start needing higher amounts of these drugs to feel the same effect (tolerance).
• Central nervous system depressants include alcohol, barbiturates (amobarbital, pentobarbital, secobarbital), benzodiazepine (Valium, Ativan, Xanax), chloral hydrate, and paraldehyde. These substances produce a sedative and anxiety-reducing effect, which can lead to dependence.
• Hallucinogens include LSD, mescaline, psilocybin ("mushrooms"), and phencyclidine (PCP or "angel dust"). They can cause people to see things that aren't there (hallucinations) and can lead to psychological dependence.
• Tetrahydrocannabinol (THC) is the active ingredient found in marijuana (cannabis) and hashish. Although used for their relaxing properties, THC-derived drugs can also lead to paranoia and anxiety.