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Tolerance
Overview
Tolerance occurs when the repeated use of a chemical substance, taken to improve mood, has a diminished effect, and more is needed to create the same mood improvement. This change, which is the brain's response to repeated substance use, is caused by the brain's ability to adapt to or compensate for the presence of a chemical.
Because tolerance develops when a person uses drugs or alcohol repeatedly, tolerance can be an early sign that the person is developing an addiction. Unfortunately, physicians rarely ask about tolerance and regularly miss the opportunity to identify early addiction. Even if a physician questions a patient about tolerance, an early diagnosis of addiction is difficult if a patient is in denial and answers dishonestly.
Some substances cause tolerance to develop more rapidly than others. When narcotics are used regularly, tolerance develops in just a few weeks, and higher doses are needed to have the same effect. The body accommodates to increased amounts of narcotics. If the narcotic is withdrawn, the body's altered chemistry is imbalanced, creating physical discomfort known as narcotic withdrawal syndrome.
Types of Alcohol Tolerance
Alcohol has different effects on various parts of the brain. Tolerance develops by different mechanisms, and its intensity varies depending on the type of mental or physical function being performed and the setting in which the alcohol is ingested.
Genetic Tolerance
Research shows that genetic factors affect the development of tolerance. Sons of alcoholics were found to be less impaired (more tolerant) to alcohol than sons of non-alcoholics. Sons of alcoholics experienced a greater effect from alcohol in the early part of a drinking session, when the pleasurable effects of alcohol predominate. Later in a drinking session, when function is most impaired by alcohol, sons of alcoholics had developed tolerance and were less impaired than sons of non-alcoholics. This genetic predisposition, which enhances the early pleasurable effects of drinking, may contribute to increased intake and may be the mechanism by which sons of alcoholics have an increased risk of developing alcoholism.
Functional Tolerance
We sometimes see very heavy drinkers who appear to function without signs of intoxication despite very heavy drinking. This is called functional tolerance. As the brain adapts to heavy alcohol use, it develops the ability to function in spite of high levels of alcohol. Since functioning is not disrupted, extremely high levels of alcohol intake are possible, often leading to damage of the liver and other body systems.
Functional tolerance develops differently, depending on the type of task involved. In one study, a group of young men developed tolerance more rapidly when they took tests that required intellectual functioning than they did when they took tests that required hand-eye coordination, such as driving a car.
Environment-Dependent Tolerance
Research has shown that when high levels of alcohol are ingested, tolerance develops independently of environmental cues. When lower levels of alcohol are ingested, tolerance can be dependent on environment cues. One study of subjects who were classified as social drinkers showed that they performed tasks better (were tolerant to the effects of the low doses of alcohol) when given alcohol in a bar-like setting than when they were given the same tasks and equal doses of alcohol in an office setting.
Learned Tolerance
Practicing a task under the influence of a substance increases the speed with which the person develops tolerance. Doing something while under the influence increases the rate at which the alcohol has a diminished effect on performance. The heavy drinker who functions normally when quite intoxicated is an example of this phenomenon.
Learned and environment-dependent tolerance can have important consequences when a person drinks and drives. Repeatedly driving the same route after ingesting low levels of alcohol could improve performance by helping tolerance to develop more rapidly. On the other hand, a driver who has ingested the same amount of alcohol but is driving a new route could lose any tolerance learned, resulting in a diminished ability to drive effectively.
Metabolic Tolerance
Repeated exposure to large amounts of alcohol causes the liver to produce greater amounts of the enzymes that metabolize alcohol. The increased metabolism of alcohol in the body reduces the duration of alcohol's intoxicating effect. This is known as metabolic tolerance. In the later stages of alcoholism, metabolic tolerance speeds up the need for greater alcohol intake, causing increased damage to the body.
Increased enzymes that metabolize alcohol also metabolize acetaminophen more rapidly, producing toxic substances that cause liver damage. This occurs after moderate social drinking and is the reason that acetaminophen pain relievers are not recommended for treating hangovers.
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