Energy drinks are popular among young teens and adults, but studies continue to show they may have unintended and potentially serious side effects, including high blood pressure, hyperactivity and more.

In a new report published in Pediatric Emergency Care, researchers conducted a questionnaire at two emergency departments from June 2011 to June 2013 that surveyed adolescents between ages 12 and 18. Of the 612 young people who responded, 33% said they frequently drank energy drinks. Among those teens, 76% said they experienced a headache in the last six months, 47% said they experienced anger and 22% reported difficulty breathing.

It’s impossible to say whether any of those behaviors were due to energy drinks, but young people who consumed them were much more likely to report the symptoms than those who didn’t. Overall, kids who consumed energy drinks often were more likely to say the drinks helped them do better in school or in sports, helped them focus and helped them stay up at night.

“Moderation is key,” says Dr. Vikas Khullar, a University of Florida fellow in Gastroenterology and Hepatology.

In a recent case study published in the journal BMJ Case Reports, Khullar and his colleagues wrote about a 50-year-old man who came to the hospital with an inflamed liver. He was in pain, vomiting and had dark urine. After running several tests for possible infections and coming up short, the doctors learned that the man drank four to five energy drinks every day for three weeks before his health issues appeared. The doctors concluded energy drinks caused his liver problems, citing another similar case that supports their suspicions. “We cannot speculate on the safety of energy drinks, however anyone with liver or heart disease should consume energy drinks with caution,” says Khullar.

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Energy drinks contain multiple stimulating ingredients, beyond caffeine. “Often energy drinks contain a energy blend which is a combination of herbal supplements as well as vitamins in often greater levels than the recommended daily intake,” he says. “Further research may be needed to determine appropriate use and dosages.”

Groups like the U.S. Center for Disease Control and Prevention (CDC) warn against mixing energy drinks and alcohol, arguing energy drinks mask the depressant effects of alcohol. Still, in a 2016 survey of 1,000 young adults, 57% said they consumed energy drinks in the past year, and 71% of those students drank energy drinks with alcohol.

As TIME has previously reported, energy drink companies insist their products are safe and that a link between their beverages and side effects can’t be confirmed. The companies also appear to be making their drinks bigger and with more sugar; Monster’s new Mutant beverages, describe as a “super soda” on the label, have now hit shelves. The 20-ounce drinks have about 70 grams of sugar (more than twice of what’s in some candy bars) and 115 milligrams of caffeine.

Groups like the Center for Science in the Public Interest have called on the U.S. Food and Drug Administration to add safety warnings to energy drinks, and American Academy of Pediatrics researchers have argued the stimulants in energy drinks have “no place in the diet of children and adolescents.”

“While more research is needed, accumulating evidence exists to suggest that energy drink consumption is linked to adverse cardiovascular events, sleep disturbances, and other substance use among adolescents,” says Amelia Arria, director of the University of Maryland School of Public Health’s Center for Young Adult Health and Development and co-author of the recent energy drink and alcohol study.

Though definitive links between the beverages and health problems are not proven, many health professionals agree: the emerging data is not encouraging.

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