What the research found

Researchers compared five-year health outcomes between insomnia patients with documented long-term melatonin use (≥12 months) and matched insomnia patients with no melatonin record. The melatonin group showed a 90% higher incidence of heart failure (4.6% vs 2.7%), nearly 3.5 times more hospitalizations for heart failure (19% vs 6.6%), and nearly double the all-cause mortality (7.8% vs 4.3%). When the analysis was tightened to include only those with repeated prescriptions across multiple months, the heart failure association weakened slightly to an 82% increase but remained substantial.

The data came from a large international electronic health records database. Participants with pre-existing heart failure diagnoses or concurrent use of other sleep medications were excluded. The researchers performed sensitivity analyses to check whether the associations held under different analytical approaches.

Why it matters for you

If you're using melatonin regularly—particularly at prescription strength or doses above typical over-the-counter formulations—this suggests you should reconsider whether chronic use aligns with your health goals. The signal here is especially relevant if you're tracking cardiovascular biomarkers (blood pressure, heart rate variability, troponin, BNP, or echocardiography findings) or have existing risk factors for heart disease.

The findings don't prove melatonin caused these outcomes, but they do highlight a gap: melatonin's long-term cardiac safety has never been rigorously studied, and people often assume it's harmless because it's "natural" and over-the-counter. If sleep is disrupted, there are better-studied alternatives—cognitive behavioural therapy for insomnia, optimizing light exposure and training timing, or discussing prescription options with your doctor that have cardiovascular safety data.

Caveats

• Observational and correlational: The study cannot prove melatonin use caused the outcomes. People choosing chronic melatonin may differ in unmeasured ways (untreated sleep apnea, metabolic syndrome, depression) that drive the risk.
• De-identified data limits context: Researchers couldn't determine country, dosage, or over-the-counter vs. prescription status for individual participants, introducing uncertainty about what was actually consumed.
• Unusual melatonin prescribing in the U.S.: Melatonin isn't FDA-approved for insomnia and is rarely prescribed long-term in the U.S., which may mean the cohort is skewed toward severe cases or countries with different use patterns.
• Preliminary, unpublished: Presented at a conference; not yet peer-reviewed or published.
• Selection bias in long-term users: People using melatonin for >1 year despite it being ineffective for many may represent a group with worse underlying health or adherence to sleep-disrupting behaviours.