Melatonin is one of the few sleep aids available over the counter in most places, which makes it an easy first thing to reach for. But "easy to buy" and "well studied over time" are different questions. Does taking it regularly eventually blunt the body's own production? And what does the evidence actually cover when people ask about using it for years, not weeks?
What a year of use looked like
The most direct long-term data comes from an open-label follow-up of 244 adults with primary insomnia who had already completed a placebo-controlled trial of prolonged-release melatonin (PRM), the depot formulation taken nightly and approved in Europe for people 55 and older. Of the 244, 112 completed 6 months and 96 completed a full 12 months of continuous treatment.
Sleep quality ratings stayed significantly better than before treatment throughout, with no evidence of tolerance building up over time. When treatment stopped after up to a year, researchers measured nocturnal urinary 6-sulfatoxymelatonin, a marker of the body's own melatonin output. There was no suppression of endogenous melatonin production, and discontinuation was not followed by rebound insomnia or withdrawal symptoms — if anything, some benefit persisted afterward.
The wider safety picture
A systematic review examined 50 controlled studies of oral melatonin that reported statistical analysis of adverse events: 26 found none that reached significance, 24 found at least one. Where effects turned up, they were generally mild, short-lived and manageable — fatigue, mood changes, and effects on psychomotor or cognitive performance were the most common. A smaller number of studies flagged effects on hormonal measures (such as reproductive parameters or glucose metabolism) and cardiovascular measures (blood pressure, heart rate), particularly tied to dose, timing, or interaction with blood pressure medication. The review's overall conclusion: a generally favorable safety profile, with known, mostly avoidable exceptions.
Where the evidence runs out
Here is the honest limit: the controlled studies behind these findings top out around 12 months. A 2025 narrative review spanning 71 articles published from 2011 to 2024 states plainly that data on melatonin's long-term effects remain limited, alongside small sample sizes and inconsistent study designs across the literature. Nobody has published a multi-year, placebo-controlled trial that would settle the question for genuinely long-term (2 or more years) use.
It's also worth separating the two common forms. Nearly all of the controlled long-term safety data concerns prolonged-release melatonin specifically. Standard immediate-release melatonin — the form most people buy over the counter without a prescription — has far less long-term, controlled follow-up behind it, even though it's the more commonly used form.
Our reading
For something this widely used, the available evidence is reassuring as far as it goes: up to a year of nightly prolonged-release melatonin showed sustained sleep benefits, no tolerance, no withdrawal, and — directly measured — no suppression of the body's own melatonin production. Its accessibility as a non-prescription option matters; it gives people a reasonably well-tolerated tool to try before reaching for something stronger. The honest caveat is that "reasonably well-tolerated" currently means "over about a year, mostly for the prolonged-release form" — genuine multi-year data, and solid long-term data on ordinary immediate-release melatonin, simply isn't there yet.
This article is for general education and is not medical advice.