A widely used over-the-counter sleep supplement may be associated with significant cardiovascular risks when used for a year or more, according to preliminary new research. The study, which analyzed health records for more than 130,000 adults with chronic insomnia, found a strong correlation between long-term melatonin use and a higher likelihood of developing heart failure, requiring hospitalization for the condition, or dying from any cause.
These findings, presented at the American Heart Association’s Scientific Sessions 2025, challenge the prevailing perception of melatonin as a harmless sleep aid and highlight the urgent need for further investigation into its long-term safety. While researchers caution that their observational study does not prove a direct cause-and-effect relationship, the results raise important questions about a supplement used by millions of Americans, often without medical supervision and with little regulatory oversight.
New Study Signals Potential Heart Risks
The recent analysis reviewed five years of electronic health records, comparing tens of thousands of insomnia patients who had used melatonin for at least 12 months with a similar group who had never taken the supplement. The results revealed a striking difference between the two groups. Those in the melatonin group had approximately 90% higher odds of being diagnosed with heart failure over the five-year period.
The data also indicated more severe outcomes were more common among melatonin users. They were nearly 3.5 times more likely to be hospitalized for heart failure and almost twice as likely to die from any cause compared to their non-user counterparts. “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,” said lead researcher Dr. Ekenedilichukwu Nnadi in a statement. Researchers emphasized that this is an “unexpected and important signal that needs to be studied further, ideally in randomized trials.”
A Complicated Biological Picture
The study’s conclusions appear to contradict a body of previous research that pointed toward melatonin having cardiovascular benefits. For years, smaller or shorter-term studies have suggested that melatonin helps regulate the cardiovascular system, primarily by reducing blood pressure and heart rate, especially at night. The hormone is known to interact with the autonomic nervous system, promoting the “rest and digest” parasympathetic activity over the “fight or flight” sympathetic system that can stress the heart.
Vascular and Blood Pressure Effects
Melatonin’s influence on blood vessels is complex and not fully understood. It acts on two primary receptors, MT1 and MT2, which can have opposite effects. Activation of MT1 receptors tends to cause vasoconstriction (a narrowing of blood vessels), while MT2 activation typically leads to vasodilation (a widening of blood vessels). This dual activity means melatonin can constrict blood flow in some parts of the body while increasing it in others, and the net effect on the cardiovascular system over the long term remains unclear. Some studies have shown low doses of melatonin can lower blood pressure, but other research using slightly higher doses found no change, suggesting the effect may be dose-dependent.
The Problem of Unregulated Supplements
Compounding the uncertainty around melatonin’s biological effects is its regulatory status in the United States. Unlike prescription medications, melatonin is classified as a dietary supplement by the Food and Drug Administration (FDA), which means it does not undergo the same rigorous testing for safety and efficacy. The FDA does not approve supplements before they are sold, and the agency has limited resources to test the vast number of products on the market.
This lack of oversight can lead to significant discrepancies between what is advertised on the label and what is in the bottle. One 2023 study that analyzed 25 over-the-counter melatonin gummy products found that 88% were inaccurately labeled. The actual quantity of melatonin ranged from 74% to 347% of the amount listed on the label. Such inconsistencies make it difficult for consumers to know what dose they are truly taking, which could be a critical factor in safety, especially with long-term use.
Expert Guidance for Consumers
In light of the new findings, experts urge caution but advise against panic, emphasizing the preliminary nature of the research. The study is an abstract that has not yet been published in a peer-reviewed journal, a critical step for scientific validation. Furthermore, as an observational study, it can only identify a correlation; it cannot rule out the possibility that other factors common to people with chronic insomnia who take melatonin are the true cause of the increased heart risks.
Rethinking Long-Term Use
The consensus among medical professionals is that melatonin is likely safe for most healthy adults for short-term use, typically defined as up to three months. However, there is very little research on the effects of taking it for longer periods. Health organizations like the American Academy of Sleep Medicine do not recommend melatonin for chronic insomnia due to insufficient evidence of its effectiveness and safety. Experts advise that anyone considering using the supplement, particularly for an extended duration or for an underlying health condition, should first consult with a health care provider. People with existing conditions such as high blood pressure, seizure disorders, or autoimmune diseases should be especially cautious.
