Energy drink usage is common and contains caffeine or other stimulants. We evaluated demographics, prevalence, reasons and adverse effects with consuming energy beverages. Cross-sectional study of a convenience sample of patients recruited from two San Diego Emergency Departments from January to December 2009. One-thousand-two-hundred-ninety-eight subjects participated of which 52.6% were male. Ethnicity: Caucasian 48.3%, African American 17%, Hispanic 18%, Other 16.7%. Age ranges: 18-29 years (38.4%), 30-54 years (49.6%) and greater than 55 years (12%). Reasons for use: 57% to "increase energy", 9.5% for studying/work projects, 2.4% while prolonged driving, improve sports performance 2%, with ethanol 6.3%, "other" reasons 22.1%. Adverse reactions reported by 33.5% (429) patients. Two-hundred-eighty report feeling "shaky/jittery", insomnia 136, palpitations 150, gastrointestinal upset 82, headache 68, chest pain 39, and seizures in 6. Eighty-five patients reported co-ingestion with illicit "stimulants" including cocaine and methamphetamine. We identified one-third of patients reported at least one adverse effect. Whilst most were not severe, a small number were serious e.g., seizures. In addition, some report purposely ingesting with illicit drugs.