By now, it is a familiar litany: Study after study suggests that alcohol in moderation may promote heart health and even ward off diabetes and dementia.
The evidence is so plentiful that some experts consider moderate drinking -- about one drink a day for women, about two for men -- a central component of a healthy lifestyle.
But what if it's all a big mistake?
For some scientists, the question will not go away. No study, these critics say, has ever proved a causal relationship between moderate drinking and lower risk of death, only that the two often go together.
It may be that moderate drinking is just something healthy people tend to do, not something that makes people healthy.
"The moderate drinkers tend to do everything right -- they exercise, they don't smoke, they eat right and they drink moderately," said Kaye Middleton Fillmore, a retired sociologist from the University of California, San Francisco, who has criticized the research. "It's very hard to disentangle all of that, and that's a real problem."
Some researchers say they are haunted by the mistakes made in studies about hormone replacement therapy, which was widely prescribed for years on the basis of observational studies similar to the kind done on alcohol. Questions have also been raised about the financial relationships that have sprung up between the alcoholic beverage industry and many academic centers, which have accepted industry money to pay for research, train students and promote their findings.
"The bottom line is there has not been a single study done on moderate alcohol consumption and mortality outcomes that is a 'gold standard' kind of study -- the kind of randomized controlled clinical trial that we would be required to have in order to approve a new pharmaceutical agent in this country," said Dr. Tim Naimi, an epidemiologist with the Centers for Disease Control and Prevention.
Even avid supporters of moderate drinking temper their recommendations with warnings about the dangers of alcohol, which has been tied to breast cancer and can lead to accidents even when consumed in small amounts, and is linked with liver disease, cancers, heart damage and strokes when consumed in larger amounts.
Health organizations have phrased their recommendations gingerly. The American Heart Association says people should not start drinking to protect themselves from heart disease. The 2005 United States dietary guidelines say that "alcohol may have beneficial effects when consumed in moderation."
Some scientists say the time has come to do a large, long-term randomized controlled clinical trial, like the ones for new drugs. One approach might be to recruit a large group of abstainers who would be randomly assigned either to get a daily dose of alcohol or not, and then closely followed for several years; another might be to recruit people who are at risk for coronary disease.
But even the experts who believe in the health benefits of alcohol say this is an implausible idea. Large randomized trials are expensive, and they might lack credibility unless they were financed by the government, which is unlikely to take on the controversy. And there are practical and ethical problems in giving alcohol to abstainers without making them aware of it and without contributing to accidents.
"The last thing we want to do as researchers and physicians is expose people to something that might harm them, and it's that fear that has prevented us from doing a trial," said Dr. Sei Lee of the University of California, San Francisco, who recently proposed a large trial on alcohol and health.
"But this is a really important question," he continued. "Because here we have a readily available and widely used substance that may actually have a significant health benefit -- but we just don't know enough to make recommendations."