Heavy drinking may be a major risk factor for early-onset dementia, new research suggests.
Scientists who looked at 57,000 cases of dementia diagnosed before the age of 65 found that 39% could be attributed to alcohol-related brain damage.
Brain damage associated with drinking was thought to be responsible for 3% of all the dementia cases investigated.
The French nationwide study involved more than a million hospital patients discharged with dementia and alcohol-related brain damage between 2008 and 2013.
Previous research has indicated that light to moderate alcohol consumption might benefit the brain. Other results have highlighted the harmful effects of drinking too much.
The World Health Organisation defines chronic heavy drinking as consuming more than 60 grams of pure alcohol per day for men and 40 grams for women.
That equates to around six or more standard drinks for men and four for women.
Writing in The Lancet Public Health journal, the study authors called for action to reduce the burden of dementia due to excessive alcohol consumption.
Lead researcher Dr Michael Schwarzinger, from the Translational Health Economics Network in France, said: "A variety of measures are needed, such as reducing availability, increasing taxation, and banning advertising and marketing of alcohol, alongside early detection and treatment of alcohol use disorders."
Excessive drinking may lead to permanent structural and functional brain damage, he pointed out. Heavy alcohol consumption could also increase the risk of high blood pressure, diabetes, stroke, disordered heart rhythm and heart failure, which may in turn increase the risk of vascular dementia.
Dr Doug Brown, from the charity Alzheimer's Society, said: "We've known for a while that heavy drinking can increase your risk of developing dementia.
"This study suggests that alcohol abuse disorders may be responsible for more cases of early-onset dementia than previously thought.
"The study doesn't change the advice to stick to no more than 14 units of alcohol a week.
"Anyone concerned about heavy drinking should visit their GP to discuss ways of cutting down and the support on offer."
Dr Sara Imarisio, from Alzheimer's Research UK, said: "As this study only looked at the people who had been admitted to hospital due to chronic heavy drinking, it doesn't reveal the full extent of the link between alcohol use and dementia risk.
"Previous research has indicated that even moderate drinking may have a negative impact on brain health and people shouldn't be under the impression that only drinking to the point of hospitalisation carries a risk."