Study details and key findings

For the study, researchers analyzed data from 130,828 adults who were diagnosed with insomnia. The average age of the participants was 55.7 years, and 61.4% were women. Of the participants, around 65,414 had been prescribed melatonin at least once and reported taking it for at least a year. Those who had prior heart failure or were taking other sleep medication were excluded.

Overall, the researchers found that participants who took melatonin for at least a year had a 90% higher chance of heart failure over five years compared to controls. In total, around 3,000 participants with long-term melatonin use experienced heart failure compared to 1,800 controls.   

Participants who took melatonin were also around 3.5 times more likely to be hospitalized for heart failure and almost twice as likely to die from any cause over five years compared to those who didn't use melatonin.

"Melatonin supplements are widely thought of as a safe and 'natural' option to support better sleep, so it was striking to see such consistent and significant increases in serious health outcomes, even after balancing for many other risk factors," Ekenedilichukwu Nnadi, chief resident in internal medicine at SUNY Downstate/Kings County Primary Care and the study's lead author.

However, Nnadi emphasized that the study only shows an association, not causation. "This doesn't prove that melatonin directly causes heart failure," he said. "It simply shows that people with chronic insomnia who took melatonin long term were more likely to experience these outcomes. It's an unexpected and important signal that needs to be studied further, ideally in randomized trials."

Commentary

So far, the study, which has not yet been peer-reviewed, has drawn mixed reactions from health experts, with many highlighting the study's limitations and saying that more research into long-term melatonin use is needed.

The researchers have acknowledged several limitations in the study, including how much melatonin participants were prescribed, how the supplement impacted their sleep habits, and whether some participants purchased melatonin over the counter instead of accessing it with a prescription. 

The study's findings also contradict previous research that suggests melatonin could be beneficial for people with heart problems. Melatonin is a well-known antioxidant and could help protect against coronary artery disease and regulate blood pressure.

"The findings are certainly provocative and warrant attention, especially given the widespread perception of melatonin as a benign, 'natural' sleep aid," Rishi said. "However, the study is observational and based on electronic health record data, which limits its ability to establish causality."

Currently, health experts recommend people use melatonin cautiously and under medical supervision, especially if they have cardiovascular risk factors.

Before using melatonin supplements, "speak to your doctor first about, for one, getting a proper diagnosis for your sleep difficulty and then discussing the appropriate course of treatment," said Marie-Pierre St-Onge, director of the Center of Excellence for Sleep & Circadian Research in the department of medicine at Columbia University Irving Medical Center. "People should be aware that (melatonin) should not be taken chronically without a proper indication."

Nnadi also encouraged people who have been taking melatonin for years to talk to their doctors about using the supplement, especially if they have heart disease or any associated risk factors.

"The takeaway isn't that melatonin is 'bad' or that everyone should stop taking it," he said. "It's that we shouldn't assume something is risk-free just because it's natural or sold over the counter."