Melatonin is a hormone available OTC in the United States and by prescription in some countries. This page summarises how its potential side effects are discussed and does not constitute medical advice.
Overview
Melatonin is generally considered well tolerated in short-term use, and its side effect profile is usually described as mild. Because it is a hormone that influences circadian timing, however, many of its effects depend heavily on when it is taken, and long-term data across diverse populations remain more limited.
Commonly reported effects
The most frequently discussed effects are generally mild and short-lived:
- Daytime drowsiness or grogginess ("hangover" feeling).
- Headache and dizziness.
- Vivid dreams or altered dreaming.
- Nausea or mild gastrointestinal upset.
Timing-dependent effects
Because melatonin acts on the circadian clock, taking it at the "wrong" time relative to a person's own rhythm can, in principle, shift timing in an unintended direction or contribute to grogginess. This is why the research literature emphasizes circadian phase so strongly, and why effects can differ substantially from one person to another.
Special populations and interactions
Several groups warrant particular caution and are commonly flagged in the literature:
- Children and adolescents, where use is more debated.
- Pregnancy and breastfeeding.
- People taking sedatives, blood-pressure medications, or other agents where interactions are discussed.
- Anyone with conditions affected by hormonal signaling.
Context and caveats
As a supplement in the US, melatonin products can vary in actual content and labeling accuracy, which adds uncertainty on top of the biology. It is a hormone rather than an inert additive, and the absence of large, long-term controlled trials in healthy people means the extended safety picture is incompletely defined. Any use should involve qualified clinical guidance, especially with other medications.