RED wine may cause worse headaches than other drinks because of a flavonol that slows the breakdown of alcohol, a study shows.
Antioxidant quercetin, most common in sun-ripened grapes, can trigger the episodes in just 30 minutes, US researchers found.
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It is broken down into compounds that block alcohol from being digested properly, resulting in a build-up of headache-causing toxin acetaldehyde.
Certain people may be more likely to react more strongly to the antioxidant, making them at greater risk of the problem, researchers suggested.
Professor Morris Levin, of the University of California, San Francisco, said: “We postulate that when susceptible people consume wine with quercetin, they develop headaches.
“We think we are finally on the right track toward explaining this millennia-old mystery.”
Quercetin is an antioxidant found in citrus fruits, apples, onions, parsley, sage, tea and red wine.
The naturally occurring chemical has been linked to a reduced risk of heart disease, high cholesterol, cancer and high blood pressure.
Professor Levin said: “Quercetin is produced by the grapes in response to sunlight.
“If you grow grapes with the clusters exposed, such as they do in the Napa Valley for their cabernets, you get much higher levels of quercetin.
“In some cases, it can be four to five times higher.”
The NHS recommends adults do not drink more than 14 units of alcohol a week, the equivalent of six pints of lager or one and a half bottles of wine.
Drinking too much booze can cause headaches the next day because it causes dehydration, one of the main culprits behind hangovers.
Red wine can also cause more immediate pains in some people, even when not drunk in high amounts, in a trend that has puzzled experts throughout history.
The study, published in the academic journal Scientific Reports, looked into why this is the case in lab tests.
Researchers tested samples of wine with different amounts of flavanols to see how they impacted levels of ALDH2 — an enzyme that helps break down alcohol in the body.
They found quercetin caused the biggest reduction ALDH2, meaning alcohol would be broken down less effectively.
Professor Levin said: “The next step is to test it scientifically on people who develop these headaches, so stay tuned.”
Professor Andrew Waterhouse, of the University of California, Davis, added: “If our hypothesis pans out, we will have the tools to start addressing these important questions.”