If you suffer pounding headaches after sipping red wine, you are not alone. It turns out many people are affected by a flavanol found in red wines that can trigger a painful, pounding headache 30 minutes to three hours after drinking.

A study by researchers at University of California, Davis found that the flavanol quercetin could be the culprit behind those headaches, says Fox News. Quercetin is found in grapes and other fruits and vegetables. It is 10 times more prevalent in red wines than white varieties.

While on its own, quercetin is healthy for you, it becomes a different animal when combined with alcohol. In fact, when it gets into your bloodstream it converts to quercetin glucuronide, which blocks the metabolism of alcohol. When alcohol is not metabolized, a toxin called acetaldehyde builds up in the system.

Researchers say that high levels of acetaldehyde can cause flushing, headaches, and nausea. And it appears that people who are more prone to migraines are more likely to experience headaches after consuming red wine. According to a university news release, the medication disulfiram prescribed to alcoholics to prevent them from drinking causes these same symptoms.

That’s because the drug also causes the toxin to build up in the body when normally an enzyme would break it down. About 40% of the East Asian population also has an enzyme that doesn’t work well, allowing acetaldehyde to build up in their system.

Experts say that sunlight increases headache-causing flavanols in grapes. Quercetin is produced by the grapes in response to sunlight, explains wine chemist and corresponding author of the study, Andrew Waterhouse, adding that in grapes grown in exposed clusters, such as those in the Napa Valley that are used to make cabernets, you get much higher levels of quercetin.

“We postulate that when susceptible people consume wine with even modest amounts of quercetin, they develop headaches, particularly if they have a preexisting migraine or other primary headache condition,” said co-author Dr. Morris Levin, professor of neurology and director of the headache center at the University of California, San Francisco. “We think we are finally on the right track explaining this millennia-old mystery. The next step is to test it scientifically on people who develop these headaches, so stay tuned.”