People with chronic insomnia who documented at least a year of melatonin use had roughly a 90 percent higher chance of developing heart failure over the following five years than matched nonusers, according to preliminary research the American Heart Association released this week. [1] Heart failure occurred in 4.6 percent of the melatonin group compared with 2.7 percent of the comparison group — a real difference, but one the study's own authors say cannot be read as proof that melatonin caused it.

The findings come from an analysis of the TriNetX Global Research Network, a de-identified international database of electronic health records. Researchers identified 130,828 adults with diagnosed insomnia, split into a "melatonin group" of 65,414 people with at least a year of documented use and a matched comparison group with no recorded use, controlling for 40 factors including age, sex, existing heart and nervous-system conditions, blood pressure, and body mass index. [1] A sensitivity check — limiting the melatonin group to people who had filled at least two prescriptions 90 days apart, to strengthen confidence they were genuinely long-term users — still found an 82 percent higher heart failure risk. Secondary outcomes were more dramatic still: hospitalization for heart failure was nearly 3.5 times as common in the melatonin group, and death from any cause was nearly twice as common. [1]

"Melatonin supplements may not be as harmless as commonly assumed," said Dr. Ekenedilichukwu Nnadi, the study's lead author and a chief resident in internal medicine at SUNY Downstate/Kings County Primary Care in Brooklyn. "If our study is confirmed, this could affect how doctors counsel patients about sleep aids." [1] But Nnadi is equally direct about what the study cannot show: "While the association we found raises safety concerns about the widely used supplement, our study cannot prove a direct cause-and-effect relationship. This means more research is needed to test melatonin's safety for the heart." [1]

The limitations are substantial enough to reshape how the headline number should be read. Melatonin regulation varies by country — it is available over the counter in the United States but requires a prescription in the United Kingdom — and researchers had no access to participants' locations, since the data was de-identified. That means an unknown number of U.S. participants who bought melatonin over the counter without it entering their medical record could have been misclassified as "nonusers," diluting any true difference between groups rather than exaggerating it. Researchers also lacked data on how severe each participant's insomnia was or whether they had co-occurring depression or anxiety — conditions that could independently drive both melatonin use and cardiovascular risk. And some of the reported "hospitalizations for heart failure" reflect broad diagnostic coding rather than a specific new-diagnosis code, meaning the hospitalization figure may overstate genuinely new cases. [1]

An outside expert raised a different concern entirely: that American physicians are prescribing melatonin for a use it was never approved for. "I'm surprised that physicians would prescribe melatonin for insomnia and have patients use it for more than 365 days, since melatonin, at least in the U.S., is not indicated for the treatment of insomnia," said Marie-Pierre St-Onge, a Columbia University sleep researcher who chaired the American Heart Association's 2025 scientific statement on sleep health and was not involved in this study. "In the U.S., melatonin can be taken as an over-the-counter supplement and people should be aware that it should not be taken chronically without a proper indication." [1] Her point cuts against the popular framing of melatonin as an inherently benign substitute for prescription sleep medication — a framing this study's own limitations do not yet settle, in either direction.

What the data supports is narrower and more useful than either the alarming headline or the reflexive dismissal: a documented association strong enough to warrant real research into causation, produced by a study too limited by its own design to prove anything about mechanism. Millions of people take melatonin nightly on the assumption that "natural" means "risk-free." This study does not prove that assumption wrong. It proves, at minimum, that nobody has yet proven it right.

-- NORA WHITFIELD, Chicago