Substance Dependence

Tolerance to the Effects of Drugs

Tolerance to drug effects and experiencing withdrawal symptoms after sudden cessation of use are characteristics of drug dependence. Tolerance to drug [Page 871]effects involves adjustments in the body to the presence of the drug. For example, metabolic tolerance may involve faster metabolism of alcohol in the liver with repeated alcohol intake. Neural adaptations that occur as a result of repetitive drug use or cellular tolerance may involve postsynaptic receptors becoming less sensitive to the drug; presynaptic sites manufacturing less of the naturally occurring neurotransmitter to compensate for the introduction of the drug that mimics its effects; or, for a drug that blocks transmission, a greater number of receptor sites being manufactured or a greater amount of the naturally occurring neurotransmitter being supplied, which could affect the lethal dose.

Withdrawal from Drug Dependency

Withdrawal symptoms vary in length and severity of withdrawal reactions from drug to drug. These symptoms consist of readjustments in physical functioning and behavior attributed to the overactivity of the nervous system. Also referred to as an "abstinence syndrome," withdrawal symptoms are experienced when physically dependent individuals stop using drugs, particularly when cessation is abrupt. Some drugs result in more severe physiological reactivity. For example, withdrawal from alcohol, sedatives, hypnotics, or anxiolytics may involve such autonomie reactivity as hand tremors, insomnia, nausea or vomiting, transient illusions or hallucinations, psychomotor agitation, anxiety, or grand mal seizures. Alternatively, stimulant withdrawal (e.g., amphetamines, cocaine) includes symptoms of fatigue or exhaustion, depression, unpleasant and vivid dreams, insomnia or hypersomnia, increased appetite, and psychomotor retardation or agitation. Opioid withdrawal includes dysphoric mood, anxiety, nausea or vomiting, muscle aches, abdominal cramping, tearing, rhinorrhea (i.e., runny nose), sweating, diarrhea, yawning, fever, and insomnia. Nicotine withdrawal includes depressed, anxious, or irritable mood; insomnia; difficulty concentrating; restlessness; decreased heart rate; constipation; sweating; and increased appetite. PCP has no or few withdrawal symptoms. Caffeine has a few withdrawal symptoms, including headaches, some fatigue, and difficulty concentrating. Not recognized until recently, cannabis also has a few withdrawal symptoms, including fatigue, difficulty concentrating, stomach pains, some agitation, perhaps anger, and vivid dreams, especially among chronic users. Hallucinogens are not known to have withdrawal symptoms, although there have been occasional reports of flashbacks (high-like states) in some people who have stopped using these drugs.

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