Alcoholic dehydrogenae is an induceable enzyme. First: that enzymes catalyses the initial reaction in metabolising ethyl [drinkable]alcohol. The more a person drinks, regularly, the more enzyme is produced, the more we can drink before getting drunk, and also suffering distressing consequences of drinkink. During the next pub outing our ingested alcohol is metabolised more quickly. Often quickly enough to lessen the symptoms from drinking. Natural basal enzyme production varies with ethnic background, gender, body weight and variety of the enzyme produces. Certain alleles produce more efficient varients of the enzyme. Women produce less alcoholic dehydrogenase than men and they usually weigh less,which makes the alcohol have a geater effect for the same amount ingested. People of European descent produce more enzyme than orientals, and native americans. The more we drink the more our metabolic efficiency rises. in other words, heavy drinkers can tolerate much higher consumption. They have to they have to increase uptake in order to get the same pleasant effect. They also metabolise the ingested alcohol more quickly. A young person would have less enzyme than us old geezers [also alcoholics]. A youngter wold commensurately get drunk faster and with less booze. Their discomfortable symptoms would also be greater. The enzyme converts alcohols to ketones and aldehydes. it works on many alcohols, including wood alcohol, [the poison methanol]. Ethanol is converted to ethyl aldehyde and that chemical is metabolised further. The more we drink; the more we can drink.

Your body would just do what it did back after you quit drinking for a few years or more (if you could do).

I would like to add to this. Not that I am contradicting any of the above, from what I understand it is true. However life is always complicated, and rarely can you look at one thing. (kidneys, liver, metabolism, body mass, fat ratios (why women tend to be more prone to getting drunk) are all variables.)Another is a variable described in psychological literature. Please, this does not mean it is not biologically based, the mind is the body.I am used to referring to it as the proponent/opponent effect (may have many names, psychology is young, and everybody keeps reinventing others' ideas, and rebranding it)Basically your body uses all its resources (some mentioned in the articles above) to balance itself. It will create an opposite effect to what it may be feeling to counter it.So, if you do herion, when you withdraw you are really excited and jumpy. Your body has been fighting the depressing effects of herion by making your body more awake. For speed adicts, they become more lethargic during withdrawal quite often, the body has built up a defence to depress itself, to counteract the effect.Same with alchohol. Part of the tolerance is various body organs becoming more effecient at disposing of the chemical. The other side is that your body will create opposite reactions to drinking, to counteract it. The more you drink, the more pronounced this becomes.Anybody aware of other relevant variables? (wild unsubstantiated opinion to follow) For instance, alchohol degrades the entire nervous system, among other things. So perhaps we are less able to feel the effects because our nervous system gets more damaged as we drink throughout our lives?

I find that as I get older, I can't drink as much - it takes me far too long to get over the after-effects. Not only do I get hangovers more easily, but a couple of glasses of wine with lunch will ruin my afternoon productivity! I still enjoy the odd drink, but I can't booze like I used to!

Most of the postings have focused on elimination of alcohol by liver metabolism, i.e. pharmacokinetics. While increases in the rate of alcohol metabolism do occur with repeated exposure of the liver to alcohol, the main reason for tolerance is probably changes in brain receptors and neurotransmitters that are affected by alcohol, i.e pharmacodynamics. For example, the acute effect of alcohol facilitates the action of the inhibitory brain neurotransmitter GABA, leading to sedation. However, over time, GABA receptors exposed to alcohol regularly can downregulate (become less sensitive to GABA), so that alcohol produces less sedation. Alcohol also affects other neurotransmitters, like glutamate, that may also contribute to tolerance. Alcohol inhibits glutamate activity acutely, but chronic alcohol upregulates/increases glutamate receptor activity. This is probably the main mechanism for tolerance to alcohol. Some of these changes are persistent and may be life long. I have seen alcohol tolerance persist for years. Even after a heavy drinker has had years of sobriety, if they return to heavy alcohol drinking, they can retain their tolerance.