Research into the popular sleeping aid, melatonin, has raised concerns about long-term usage of the supplement. The study, which has not yet been peer-reviewed, found that those prescribed melatonin for over a year are at greater risk of heart issues and are twice likely to die from any cause from these causes compared to those not prescribed it. The results suggest that longer-term uses of melatonin need to be studied further to ensure they are safe. The research was presented at the American Heart Association's Scientific Sessions and may affect how doctors counsel patients about sleep aids. Melatonin supplements are currently available over the counter in the US and the UK, where melatonin is only available by prescription.