Energy drinks are raising alertness as well as alarm. Between 2007 and 2011, the number of ER visits related to consuming highly caffeinated energy drinks more than doubled from 10,000 to nearly 21,000, according to a new report.

Patients are complaining about insomnia, nervousness, headaches, and fast heartbeat. In 42 percent of cases, alcohol or drug use were also reported, leaving 58 percent of visits related to the energy drinks alone.

Men are more likely to visit the ER for energy drink–related complaints, and people ages 18 to 25 are the most frequent visitors by age.

Energy drinks contain high amounts of caffeine and can range from about the same amount as in one 5-ounce cup of coffee to about five 5-ounce cups of coffee. The combination of caffeine and other stimulant ingredients can boost both the central nervous system and the cardiovascular system to dangerous levels, especially if someone already has a health condition.

The U.S. Food and Drug Administration has responded to concerns about energy drinks and reportedly will be reviewing their safety this spring.