In a small study of 44 healthy young adults at McGovern Medical School at UTHealth Houston, the arteries' ability to widen had roughly halved 90 minutes after a single 24-ounce energy drink. Cardiologist John Higgins, M.D., and colleagues presented the findings at the American Heart Association Scientific Sessions in 2018.
Flow-mediated dilation, an ultrasound measure of how well artery walls relax, fell from 5.1 percent at baseline to 2.8 percent after the drink. A sudden drop is one of the earliest signals of vascular stress.
The finding does not stand alone. On July 20, 2026, the American Heart Association issued a scientific statement that puts energy drinks in a separate risk category from ordinary coffee.
Why the drink hits harder than coffee
Energy shots pack 40 to 69 mg of caffeine per fluid ounce, three to four times the concentration of regular brewed coffee, according to the AHA statement. Most cans also carry taurine, guarana, and sugar.
Also read: Five a day may be enough: Study finds surprising effect on bones
The statement flagged the ingredient combination rather than caffeine alone. It noted that very high doses have been "linked to abnormal heart rhythm in people usually considered to have low cardiovascular disease risk, such as healthy adults younger than age 30."
What the wider evidence shows
A systematic review published in Current Cardiology Reports in November 2025 pooled 37 trials covering 1,597 people, with a mean age of 22.6 years. Heart rate rose in 60.9 percent of the studies, and blood pressure climbed in most as well.
Changes to the ECG, the electrical tracing of the heartbeat, followed a similar pattern. Roughly two thirds of studies measuring the QTc interval, a marker of the heart's electrical reset time, found it lengthened after consumption.
QTc prolongation is a known trigger for dangerous arrhythmias, including torsades de pointes, a life-threatening rhythm disturbance. Effects were documented for up to four hours after a drink.
Also read: A specific type of deep sleep may help shield your brain from Alzheimer's damage
Who faces the greatest danger
The review named several vulnerable groups, including people with hypertension, obesity, and existing arrhythmias. Women may face a greater reduction in cerebral blood flow than men, the authors noted.
For healthy adults, the FDA and the AHA statement place the daily caffeine ceiling at about 400 mg, roughly four to five cups of brewed coffee.
Gregory M. Marcus, M.D., chair of the AHA statement's writing group and a cardiology professor at UC San Francisco, framed the practical line for consumers.
"High doses of caffeine, such as those found in energy drinks including energy shots, may have harmful effects on the heart and should be avoided," he said.
Also read: Dietitians point to this snack to boost your brain health
Anyone with a diagnosed heart rhythm problem, poorly controlled high blood pressure, or a pregnancy should talk with their doctor before adding energy drinks to a normal day.
Also read: 7 warning signs of esophageal cancer that are easy to write off as heartburn
This article is made and published by Emil Martesen, who may have used AI in the preparation.