Does using melatonin raise heart failure risk? A preliminary (not peer-reviewed) conference abstract reported that adults with insomnia with ≥1 year of documented melatonin use had a 90% higher 5-year rate of heart failure and 3.5x more hospitalizations than adults without a documented use of melatonin. There are some major caveats: this is observational (an association, not causation), the database mixed countries where melatonin is over-the-counter vs prescription, and insomnia severity and other psychiatric conditions weren’t captured. Importantly, randomized trials in people with established heart failure show no harm from melatonin use and some signals of benefit including better quality of life and improved ejection fraction. This is an interesting real-world signal that merits peer review and careful follow-up, but it is not proof that melatonin causes heart failure.