Foreword: This topic is tricky because some people have some self-interest in it. Lack of honesty of some sources is to be feared: against alcohol from prohibitionists, in its favor from companies that live off it and drunkards that die off it.
A survey from the "Health Education Authority in Britain" showed that more than half the population believed that alcohol is good for their health. And about 25 % of drinkers are not aware of the hazard. [Lee N. 1999].
How much alcohol is too much? It depends on who is drinking and what the circumstances are. For instance, a pregnant woman or somebody on certain kinds of medication should not drink at all.
The amount of alcohol in the blood system after the consumption of alcohol depends on the quantity and the kind of drink, on the weight and gender of the drinker. The weight matters because of dilution effects. A man and a woman having the same weight will not react the same: the woman will not tolerate alcohol as much as the man. A possible explanation is that women may metabolize alcohol a different way. Another possible explanation is the proportion of fat and water which is different for men and women.
Some medications do not mix with alcohol. It is then stated on the package or within the directions. Interactions can be of three kinds:
The Deutsches Institut für Ernärhungsmedizin und Diätetik (D.I.E.T.) (cited in "Ärtze Zeitung" February 11th 2002) sums up common health tips for drinking. They advise not to drink alcohol with an empty stomach: it is a good idea to eat food containing vegetal fat which coats and protects the stomach. Drink water to avoid dehydration. Eat salty food to balance the loss of salt associated with alcohol consumption. It is also advised to avoid drinking many different kinds of drinks. The next day... have a good breakfast, including dairy products, orange juice et mineralized water.
Effects of moderate amounts of alcohol are not well known, but excess is well known to be detrimental: Gutjahr [Gutjahr et al. European addiction research 7, 117 (2001)] listed no less than 60 effects.
Excessive alcohol consumption, if it lasts, can overwhelm the regenerating capabilities of the liver, causing fat liver disease, alcoholic hepatitis and cirrhosis.
It is quite sensitive to alcohol which reduces its activity (may go down to coma or death.) Long exposures to alcohol can create an addiction: the brain then needs a given amount of alcohol every day to maintain its activity. At long times, such excess can lead to the loss of brain cells. A side effect is the deterioration of certain parts of the brain (frontal and temporal lobes) which decreases self-control and impairs judgment; this leads to an increase in the risk of violent behavior [Amen 2000].
Alcohol reduces bone density and increases their brittleness (osteopenia) [NIAAA 2000]. Another effect is that consuming alcohol increases the risk of injuries through falls. It is worth noting that the skeleton is especially exposed during growth.
Effects in men include impotence, sterility and testicular atrophy. In women, periods can be disturbed and the risk of spontaneous abortion and premature menopause rise. [NIAAA 2001, Kesmodel et al 2002]
Recent studies found that alcohol could be protective. These studies have been made widely known but their results generally still have to be reproduced. It would therefore be hazardous to draw definitive conclusions from them. It is also hard to tell whether some of these effects are linked to alcohol or not. Many studies showed a correlation between alcohol and tobacco consumption and with other factors which may dominate the effects of alcohol. Wannamethee and Shaper [American journal of public health 89, 685 (1999)], who found that a moderate consumption of alcohol was correlated to a lower death rate than that of the overall population, are careful: "A large part but not all of the greater benefit seen in wine drinkers relative to other drinkers can be attributed to advantageous lifestyle characteristics." Indeed drinkers of wine are generally more educated, have a better socio-economic statute, a higher I.Q. and are socially more integrated than drinkers of beer or spirits [Mortenen et al. Archive of internal medecine 161, 1844 (2001)]. Non-drinkers can feel cast out from societies where the consumption of alcohol is common, this may cause stress and anxiety which may be directly responsible for their health. [Pittmnan 1995, Andreasson 1998, Green 2001]
Japanese researchers from the National Institute of Sciences on Longevity in Aichi prefecture measured the I.Q. of 2000 people aged between 40 and 70. Moderate drinkers have an I.Q higher than that of non-drinkers (3.3 points for men and 2.5 for women.) The kind of alcohol drunk did not matter. Subjects had been chosen so as to exclude the influence of education and income. Researchers noticed for instance that drinkers of sake also eat raw fish which can bring fatty acid to the brain. If the effect of alcohol is real, polyphenols are thought to be responsible for it.
Reference: New Scientist, December 9th 2000.