People who regularly take melatonin to improve sleep may face serious health risks.
A preliminary study presented at the American Heart Association's Scientific Sessions 2025, Nov. 7-10 in New Orleans, found that adults with chronic insomnia who used melatonin for one year or longer were more likely to develop heart failure, be hospitalized for heart failure and die from any cause than those who did not take the sleep supplement.
What is melatonin?
Melatonin is a hormone produced by the pineal gland that regulates the body's sleep-wake cycle. Its levels naturally rise in the dark and drop during daylight hours.
Synthetic melatonin, which is chemically identical to the natural hormone, is widely used to treat insomnia (difficulty falling and/or staying asleep) and jet lag.
In many countries, including the U.S., melatonin supplements can be purchased over the counter. However, because they are not regulated in the U.S., products can differ widely in purity and dosage.
About the new study
Researchers divided 13,828 participants with insomnia into two groups based on their medical records. Those who had taken melatonin for at least one year were classified in the "melatonin group," while individuals with no record of melatonin use were placed in the "non-melatonin group."
"Melatonin supplements may not be as harmless as commonly assumed. If our study is confirmed, this could affect how doctors counsel patients about sleep aids," said the lead author of the study.
Investigating heart failure and melatonin safety
Little evidence exists on melatonin’s long-term cardiovascular effects. As such, the research team wanted to know whether long-term use could influence heart failure risk in people with chronic insomnia. Heart failure occurs when the heart cannot pump enough oxygen-rich blood to sustain the body's organs. The condition affects about 6.7 million U.S. adults.
To explore this question, scientists used data from the TriNetX Global Research Network, an international database of de-identified medical records.
They reviewed five years of data on adults diagnosed with chronic insomnia who had documented melatonin use for more than one year. Each was matched with another person who also had insomnia but had never used melatonin.
Individuals with a previous diagnosis of heart failure or who had been prescribed other sleep medications were excluded from the study.
The melatonin and non-melatonin groups were matched for age, sex, race/ethnicity, heart and nervous system diseases, medications for heart and nervous system diseases, blood pressure and body mass index. Researchers looked at electronic medical records from the five years after the matching date.
The findings
The main analysis found that among adults with insomnia, those whose electronic health records indicated long-term melatonin use (12 months or more) had about a 90% higher chance of incident heart failure over 5 years compared with matched non-users (4.6% vs. 2.7%, respectively).
There was a similar result (82% higher) when researchers analyzed people who had at least 2 melatonin prescriptions filled at least 90 days apart. (Melatonin is only available by prescription in the United Kingdom.)
A secondary analysis found that participants taking melatonin were nearly 3.5 times as likely to be hospitalized for heart failure when compared to those not taking melatonin (19.0% vs. 6.6%, respectively).
Participants in the melatonin group were nearly twice as likely to die from any cause than those in the non-melatonin group (7.8% vs. 4.3%, respectively) over the 5-year period.
The researchers accounted for many other risk factors.
What experts say
"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. 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," said 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, Multidimensional Sleep Health: Definitions and Implications for Cardiometabolic Health. St-Onge, who was not involved in this study, is a professor of nutritional medicine in the division of general medicine and director of the Center of Excellence for Sleep & Circadian Research in the department of medicine at Columbia University Irving Medical Center in New York City.
Study limitations
The study has several limitations. First, the database includes countries that require a prescription for melatonin (such as the United Kingdom) and countries that don't (such as the United States), and patient locations were not part of the de-identified data available to the researchers.
Since melatonin use in the study was based only on those identified from medication entries in the electronic health record, everyone taking it as an over-the-counter supplement in the U.S. or other countries that don't require a prescription would have been in the non-melatonin group; therefore, the analyses may not accurately reflect this.
The researchers also lacked information on the severity of insomnia and the presence of other psychiatric disorders. “Worse insomnia, depression/anxiety or the use of other sleep-enhancing medicines might be linked to both melatonin use and heart risk," the researchers said.
As well, the study cannot prove a direct cause-and-effect relationship. More research is needed to test melatonin's safety for the heart.
Source: American Heart Association
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