biological, clinical, and population studies—paints a reasonably detailed picture of the consequences of chronic marijuana use.

Because it is illegal, some people equate any use of marijuana with abuse. The IOM team chose instead to apply the definition of substance abuse used by the medical profession: that people who abuse marijuana use it repeatedly and to their personal detriment. This is the essence of substance abuse as described by the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV),15 the most widely used diagnostic system for mental health care. When people use marijuana compulsively and have trouble stopping despite the fact that their behavior causes severe problems, their diagnosis is more serious than abuse. The DSM-IV classifies such behavior as substance dependence (see Box 3.1).

THE PHYSIOLOGY OF USE AND ABUSE

Even marijuana users who do not fit the DSM-IV criteria for abuse or substance dependence may experience symptoms of tolerance, physical dependence, and withdrawal. Tolerance, a common response to the repeated use of any drug, occurs when increasing amounts are required to produce a given effect. Physical dependence describes the body's adaptation to frequently used drugs. While physical dependence can lead to substance abuse, it does not necessarily do so. If someone who is physically dependent on a drug stops taking it, he or she is likely to experience withdrawal symptoms. Most drugs that are abused produce tolerance, physical dependence, and withdrawal—but so do caffeine and nicotine as well as many nonaddictive drugs for pain, anxiety, and high blood pressure. For example, if people who take the medication propranolol for hypertension abruptly stop taking the drug, they are likely to experience withdrawal symptoms that include a temporary rise in blood pressure. To avoid these problems, patients must gradually decrease their dose of propranolol before switching to a different hypertension drug.

Regular marijuana users quickly develop tolerance to most of the drug's effects. This may be why heavy users appear to be less impaired than light users after smoking similar amounts of marijuana, despite the finding that heavy users tend to accumulate higher levels of THC in their blood. If users go without marijuana



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