The comforting belief among drinkers that a little bit of what they fancy also does them good is being challenged by doctors after a quarter of a century in which medical opinion supported the idea that alcohol in moderation protected their hearts.

Public health specialists today say the evidence that light to moderate drinking reduces the chance of failures in the blood supply may not be as strong as it once seemed. People should "not assume that there is a window in which the health benefits of alcohol are greater than the harms - there is probably no free lunch", say a team of researchers from Auckland University, in a commentary in the Lancet.

Doctors may have been blinded by plausible explanations of the effect of alcohol in blood - that it raises "good" cholesterol levels and thus protects the arteries, or that it has aspirin-like qualities in dealing with blood clots.

But Dr Rod Jackson and colleagues maintain that evidence of heart protection is more convincing for heavy drinkers. Postmortem studies have shown that dead alcoholics have relatively clean arteries, but for this group the dangers of excessive alcohol clearly outweigh any benefits.

Studies since the 1970s have laid the groundwork for drinkers, particularly those of red wine, to feel their tipple might be medicinal. Pooling of early research suggested a 20% to 25% reduction in heart disease risk linked to between one and three standard alcoholic drinks containing about 10g of alcohol each day.

But such rare good news from medical research may have been a little premature. The Auckland researchers say positive messages may have been due to other factors. For instance, people who stop drinking because of heart problems may be misclassified as "never drinkers". A big study earlier this year on 200,000 adults in the US found 27 of 30 cardiovascular risk factors were far more common in non-drinkers than in light to moderate drinkers - men who consumed two drinks a day or less and women who had one drink a day or less. These could distort results.

The British Heart Foundation had some good cheer. Belinda Linden, head of medical information, said: "There is no evidence to suggest light to moderate alcohol consumption will actually harm the heart. But over-indulging can have an adverse effect on your health."