One of the most popular supplements sold in the US could be having an unintended side effect, research suggests.
Often promoted as a safe sleep aid, melatonin is a hormone-based supplement that many people can buy over the counter and take without much thought.
But preliminary research has some scientists worried about what happens when taking the supplement becomes a long-term habit.
A recent analysis of adults with insomnia has linked being prescribed melatonin for more than a year to a greater risk of heart issues.
Among more than 130,000 adults across multiple countries, those who were prescribed melatonin long-term were more at risk of heart failure and death in the following five years than those who were not prescribed it.
It found that people prescribed melatonin for more than a year had an 89 percent higher risk of heart failure over five years, and were about twice as likely to die from any cause.
Watch the clip below for a summary of the findings:
The research, which has not yet been peer-reviewed, was presented last November at the American Heart Association's Scientific Sessions.
The findings are preliminary and don't change current health recommendations.
They do not prove that melatonin – the fourth most popular natural product taken by adults in the US – is causing dangerous repercussions.
But they do suggest that longer-term use needs closer study.
Melatonin is typically considered safe and well-tolerated for short-term use among those who are not pregnant or breastfeeding. That means around 1 to 2 months.
There are limited studies on the effects after that time, a situation that some researchers believe should be remedied, especially as the supplement grows in popularity.
"Melatonin supplements may not be as harmless as commonly assumed," medical scientist Ekenedilichukwu Nnadi at SUNY Downstate/Kings County Primary Care in New York said.
A melatonin supplement replicates a hormone naturally produced by the brain to help keep the body's internal clock ticking. Taking this substitute at the end of the day can help some people fall asleep and stay asleep.
In the US and many other nations, melatonin supplements are available over the counter, meaning patients can take them without medical guidance or supervision regarding dosage or duration of use.
While some studies suggest that long-term melatonin use is safe, comprehensive research is lacking.
In the recent analysis, Nnadi and colleagues examined electronic health records from adults in the US and multiple other countries, including the UK, where melatonin is available only by prescription.
A secondary analysis found that those who took melatonin for over a year were almost 3.5 times as likely to be hospitalized for heart failure when compared to those not taking melatonin.
Plus, the risk of dying from any cause jumped from 4.3 percent to 7.8 percent between groups.
"Melatonin supplements are widely thought of as a safe and 'natural' option to support better sleep," said Nnadi.
"So it was striking to see such consistent and significant increases in serious health outcomes, even after balancing for many other risk factors."
The findings are concerning, but they should be interpreted with caution. The study's methodology has a major limitation.
Participants were not surveyed on their melatonin use.
Instead, prescription records were used to estimate who was taking the supplement.
That means the control group, made up of those who were not prescribed melatonin, may have included people in the US who bought melatonin over the counter and took it without a prescription.
Carlos Egea, President of the Spanish Federation of Sleep Medicine Societies, who was not involved in the study, noted this caveat in an expert reaction to the research.
However, he said the associations should be taken seriously.
"These findings challenge the perception of melatonin as a benign chronic therapy and only highlight the need for a prospective trial with a control group to clarify its safety profile," argued Egea.
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Even in the short term, melatonin can come with downsides that many users may not know about.
For instance, a series of non-fatal overdoses among children in Australia has raised questions about the safety profile of the supplement for certain age groups.
Melatonin products may be based on a natural hormone, but that does not mean they are risk-free.
"If our study is confirmed, this could affect how doctors counsel patients about sleep aids," said Nnadi.
The findings were presented at the American Heart Association's Scientific Sessions.
An earlier version of this article was published in November 2025.