Does long-term melatonin use carry risks? The honest answer is we don't know

Melatonin is the sleep aid people reach for precisely because it feels harmless — a natural hormone, sold over the counter, taken nightly by millions. Ask whether long-term use carries risks and the honest answer is uncomfortable: nobody really knows, because the trials that would tell us have barely been done [s1]. What the evidence does show is that short courses are generally well tolerated, but that two separate, better-documented problems deserve more attention than the reassuring packaging suggests — inconsistent supplement content, and a steep rise in accidental poisonings among children [s2][s3].

The safety data run out fast

The most thorough review of melatonin's side effects pooled 37 randomised, placebo-controlled trials in people with sleep disorders [s1]. Daily doses ranged from 0.15 mg to 12 mg, and the reassuring headline is that adverse events were few and mostly mild: the most frequently reported were daytime sleepiness (1.66%), headache (0.74%), other sleep-related effects (0.74%), dizziness (0.74%) and hypothermia (0.62%), with no events judged life-threatening [s1].

The catch is buried in the methods. Participants were monitored for up to 29 weeks, but most studies lasted four weeks or less [s1]. The review's own conclusion is blunt: the scarcity of evidence from long-term trials limits any conclusion about the safety of continuous melatonin over extended periods, and there are insufficient robust data to judge concerns about at-risk groups such as children or people with epilepsy or asthma [s1]. Study quality was uneven, too — only eight trials were rated "good" for risk of bias, against 16 "fair" and 13 "poor" [s1]. So the familiar claim that melatonin is "proven safe" overstates a body of evidence built mostly on short courses in adults.

You may not be taking what the label says

A distinct problem is that melatonin in most countries is regulated as a dietary supplement, not a medicine, and the contents are not tightly controlled. When researchers chemically analysed 31 commercial melatonin products, the actual melatonin content ranged from 83% below to 478% above what the label claimed [s2]. More than 71% of the supplements failed to match their label within a 10% margin, and lot-to-lot variability within a single product reached 465% [s2].

Worse, a further 26% of the products contained serotonin — a related, biologically active compound and controlled substance — at levels of 1 to 75 micrograms [s2]. That means a consumer choosing a "5 mg" tablet may be getting a small fraction of that dose one night and several times it the next, sometimes with an unlabelled contaminant. For a substance whose long-term effects are already poorly characterised, not knowing the real dose compounds the uncertainty.

The clearest harm signal is in children

If there is a place where melatonin's risks have become measurable, it is paediatric poison-control data. A report drawing on the United States National Poison Data System found 260,435 paediatric melatonin ingestions reported over 2012 to 2021, with the annual number rising 530% across the decade [s3]. Melatonin ingestions grew from 0.6% of all paediatric ingestions reported to poison centres in 2012 to 4.9% in 2021 [s3]. Most were unintentional, in children aged five and under, tracking the supplement's soaring popularity — sales rose from $285 million in 2016 to $821 million in 2020 [s3].

The great majority of these ingestions caused no or minor symptoms, but the tail was serious: over the study period five children required mechanical ventilation and two died [s3]. That is not an argument that a labelled adult dose is dangerous to an adult; it is a reminder that gummy formulations marketed as benign are attractive to small children and are being consumed in a fast-growing, loosely supervised market.

What the evidence supports

Taken together, the picture is neither alarmist nor reassuring in the way the marketing implies. For occasional, short-term use in adults, melatonin looks well tolerated in the trials that exist [s1]. But the long-term safety question is genuinely open, the product you buy may not contain the dose it claims, and paediatric exposures are climbing [s1][s2][s3]. Those are reasons to treat melatonin as a medicine with real uncertainties rather than a free pass, to keep it away from children, and — because dosing and safety are involved — to raise ongoing use with a clinician rather than a supplement aisle. This is an informational summary, not medical advice.

Sources

• Adverse Events Associated with Melatonin for the Treatment of Primary or Secondary Sleep Disorders: A Systematic Review — CNS Drugs, 2019-11-04
• Melatonin Natural Health Products and Supplements: Presence of Serotonin and Significant Variability of Melatonin Content — Journal of Clinical Sleep Medicine, 2017-02-15
• Pediatric Melatonin Ingestions — United States, 2012–2021 — MMWR Morbidity and Mortality Weekly Report, 2022-06-03