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Tolerance may be defined as a drug producing a decreasing effect with repeated doses, or when larger doses of a drug must be administered to produce the same effect.¹

Tolerance to alcohol’s effects influences drinking behaviour and drinking consequences in several ways. Several types of tolerance may be distinguished by their different mechanisms. They include acute tolerance, environment-dependent tolerance and learned tolerance. However, the two main types are metabolic and functional tolerance:

• Metabolic tolerance occurs in response to the increase in consumption of alcohol. The liver increases its production of the enzyme alcohol dehydrogenase, which in turn, leads to faster metabolism of the alcohol, a lower peak blood alcohol concentration (BAC) and a faster elimination process. This reduces the duration of alcohol’s intoxicating effects. An increase in the production of alcohol dehydrogenase and its corresponding increase in liver activity harms the liver.

• Functional tolerance occurs when the brain adapts to compensate for the disruption caused by alcohol to behaviour and body functions. This type of tolerance may be seen in chronic heavy drinkers who show few obvious signs of intoxication with high BAC. Because the drinker does not experience significant behavioural impairment as a result of drinking, tolerance may facilitate the consumption of an increasing amount of alcohol. Although a person's sensitivity to alcohol's effects may be decreased, their BAC continues to rise with increase in alcohol consumption.

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