Adults with chronic insomnia who took melatonin for at least a year were about 90% more likely to develop heart failure over the next five years, according to preliminary research presented at the American Heart Association’s Scientific Sessions 2025. The findings do not prove that melatonin itself caused the increased risk, but they raise new questions about the long term safety of a supplement widely used as a sleep aid.
Researchers analyzed electronic health records from the TriNetX Global Research Network, which contains de identified patient data from multiple countries. They identified 130,828 adults diagnosed with insomnia, including 65,414 who had been prescribed melatonin for at least 12 months. The remaining participants had no recorded melatonin use. Anyone with a prior diagnosis of heart failure or who had been prescribed other sleep medications like benzodiazepines was excluded from the study.
After matching the groups for age, sex, race, existing health conditions, and medications, the researchers found that 4.6% of those who took melatonin developed heart failure within five years, compared with 2.7% of those who did not. The risk of hospitalization for heart failure was nearly 3.5 times higher in the melatonin group, with 19.0% hospitalized versus 6.6% in the comparison group. Death from any cause was also more common among melatonin users, with 7.8% dying during the five year period compared with 4.3% in the non melatonin group.
Ekenedilichukwu Nnadi, M.D., lead author of the study and chief resident in internal medicine at SUNY Downstate/Kings County Primary Care in Brooklyn, New York, said the results challenge the assumption that melatonin is a harmless sleep aid. “Melatonin supplements may not be as harmless as commonly assumed,” Nnadi said. “If our study is confirmed, this could affect how doctors counsel patients about sleep aids.”
The study’s findings were further scrutinized by Marie Pierre St Onge, Ph.D., C.C.S.H., FAHA, chair of the writing group for the American Heart Association’s 2025 scientific statement on sleep health. St Onge, who was not involved in the research, noted that melatonin is not indicated for long term use in the U.S., where it is available over the counter. “I’m surprised that physicians would prescribe melatonin for insomnia and have patients use it for more than 365 days,” she said. “People should be aware that it should not be taken chronically without a proper indication.”
The researchers emphasized that their study was observational and cannot prove that melatonin directly caused the increased risks. They also highlighted several limitations, including the lack of data on where participants lived, how severe their insomnia was, or whether they had other psychiatric conditions. Melatonin use was only identified when it appeared in medical records, meaning some over the counter users may have been misclassified as nonusers. The study authors called for further research to determine whether long term melatonin use affects cardiovascular health and, if so, why.
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