pacity for abuse. But ultimately the better a drug makes people feel, the more likely they are to abuse it. This effect, called reinforcement, generally depends on drug dosage. Caffeine, for example, is reinforcing for many people who drink a cup or two of coffee at a time but is aversive—that is, it makes most people feel worse, not better—if they consume the caffeine equivalent of six cups of coffee all at once. Reinforcement for a particular drug also varies from person to person. In the case of caffeine, research indicates that its effects are the most pleasurable for the least anxious people.

Marijuana is indisputably reinforcing to many people. Some have argued that marijuana has a relatively low potential for abuse, based on experiments in which animals—who willingly dose themselves with cocaine —did not self-administer THC. Other studies indicate that THC is rewarding to animals in relatively mild doses but that, like many reinforcing drugs, it is aversive in large amounts. Cannabinoids have also been shown to unleash a surge of dopamine, a chemical generally associated with reinforcement, in rats; however, the mechanism by which cannabinoids exert this effect appears to be different from that of other abused drugs such as cocaine and heroin. It is also important to note that the dopamine “reward” system in the brain responds to a wide variety of stimuli, not all of which are dangerous substances. For example, from animal studies we know that dopamine levels also rise in response to feelings of sexual attraction and when eating sweet foods. Based on similarities in brain structure and function, this is probably true of humans as well.

As people progress from tolerance to physical dependence to drug abuse, their craving intensifies despite mounting problems caused by their behavior. This intense desire for a drug is the toughest part of addiction to overcome. As a result, most recovering addicts suffer a relapse within one year of becoming drug-free. Animal studies suggest that this tendency to relapse results from long-term changes in brain function brought on by addiction. These alterations appear to persist for months or years after the last use of an addictive drug.

Anticraving medications have been developed for nicotine and alcohol, while methadone reduces cravings for heroin as it blocks the drug 's euphoric effects.

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