Long-term melatonin use may be far less harmless than we thought! A large study of more than 130,000 people with chronic insomnia found that those using melatonin for at least a year had a 90% higher risk of developing heart failure over the following five years. Heart failure occurred in 4.6% of long-term users vs. 2.7% of non-users. Even more striking: melatonin users were around 3.5x more likely to be hospitalized for heart failure, while all-cause mortality was 7.8% vs. 4.3%. Important caveat: this was an observational study, so it cannot show that melatonin itself caused these outcomes. But given how casually millions of people take melatonin for months or even years, this definitely deserves more research.