Alcohol Tolerance

Tolerance To Alcohol

Chronic heavy drinkers usually develop some tolerance to alcohol, which means they need to drink more alcohol to achieve the same intoxicating effect.

Functional Tolerance

With continuous alcohol use, the brain becomes less sensitive to alcohol, so increasing amounts are needed to achieve the same effect. Adolescents may have higher tolerance to depressant and other unpleasant effects of alcohol than adults ^[1].

Metabolic Tolerance

Chronic heavy drinkers may eliminate alcohol 2 or, rarely, up to 4 times as fast than moderate drinkers and therefore need double or greater amount of alcohol to maintain the same blood alcohol concentration (BAC) ^[2].

Heavy drinking (>0.1-0.2 BAC) activates additional liver enzymes called Microsomal Enzyme Oxidising System (MEOS), consisting of cytochrome P450 2E1 ( CYP2E1) and catalase that break down alcohol, so in chronic alcoholics alcohol may be eliminated from the blood up to 4 times faster than in occasional drinkers ^[9].

When a chronic alcoholic starts to drink quickly (several drinks in 1 hour) he or she will achieve about the same initial BAC as a moderate drinker of the same weight who drinks the same amount of alcohol in the same time, though ^[3]. Heavy drinkers who have developed advanced liver cirrhosis produce smaller amounts of enzymes that break down alcohol, so they may reach higher BACs after the same amount of alcohol as they did until they have healthy livers ^[5-p.176]. This is known as reverse tolerance ^[6].

After stopping regular heavy drinking, tolerance to alcohol will slowly decrease, which is known as healthy tolerance reversal, and after starting drinking heavily and regularly again it will develop faster than the first time ^[3].

Behavioral or Learned Tolerance

Chronic heavy drinkers may learn how to perform certain repetitive tasks, such as driving, without much difficulty, until driving in the same environment and until something unexpected happens ^[7,8]. A chronic heavy drinker may be able to control his or her body after drinking a certain amount of alcohol much more than an occasional drinker because of behavioral tolerance.

Environment-dependent intolerance. When someone is drinking in a new environment to which is not accustomed, his or her tolerance to alcohol may be much smaller than drinking in a known environment and he or she may become disabled after much lower dose of alcohol than usually ^[5-p.176].

Behavioral and functional cross-tolerance between alcohol and barbiturates ^[11], benzodiazepines (diazepam, altrazolam) ^[12] and inhalation anesthetics ^[13] may develop, but less likely between alcohol and opiates (morphine, codeine, heroin) ^[12], cannabis (marijuana) ^[10] or stimulant drugs, such as amphetamines or cocaine.

Genetics and Tolerance to Alcohol

Some aspects of tolerance to alcohol may be genetically determined ^[8].

Acute Tolerance to Alcohol – the Mellanby Effect

At a certain blood alcohol concentration (BAC), for example at 0.08 g/100 mL, you may be more affected when your blood alcohol concentration (BAC) has raised from zero to 0.08, than when it has fallen, for example, from 0.12 to 0.08 ^[3,4]. This is because your brain may acquire some acute tolerance to alcohol’s effects during a single drinking session ^[3,4]. Acute tolerance may develop only to certain alcohol effects, such as feeling of intoxication, and not necessary to impaired coordination and other body functions ^[8]. Acute tolerance to alcohol tends to be greater in light than in heavy drinkers ^[2-p.387].

Sensitization to Alcohol

Repeated occasional (but not regular) excessive drinking may result in sensitization, which means the same amount of alcohol affect you more than it did before you started to drink excessively. Sensitization to alcohol is the opposite of tolerance to alcohol.

Behavioral sensitization is a suggested mechanism due to which a person who drinks heavily and repeatedly experiences more and more pleasurable effects of alcohol. Behavioral sensitization in the presence of environmental cues can result in the switch from the controlled to uncontrolled drinking ^[15].