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, the 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 psychological 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.

Signs of drug use in children include but are not limited to:

• A change in the child's friends
• Withdrawn behavior
• Long unexplained periods away from home
• Lying
• Stealing
• Involvement with the law
• Problems with family relations
• Acting drunk or high (intoxicated), confused, impossible to understand, or unconscious
• Distinct changes in behavior and normal attitude
• Decreased school performance

• Opiates and narcotics are powerful painkillers that cause drowsiness (sedation) and feelings of euphoria. These include heroin, opium, codeine, meperidine (Demerol), hydromorphone (Dilaudid), and 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 barbiturates (amobarbital, pentobarbital, secobarbital), benzodiazepine (Valium, Ativan, Xanax), chloral hydrate, and paraldehyde. The most commonly used, by far, is alcohol. These substances produce a soothing sedative and anxiety-reducing effect and 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.

People may become high on drugs or overdose by accident or on purpose. Drug withdrawal symptoms can occur when a person stops or reduces their use of a substance. Withdrawal symptoms vary, depending on the abused substance. When withdrawal symptoms begin depends on the length of time the drug normally stays within the body. Drug intoxication, overdose, and withdrawal can sometimes be life-threatening.