Taking melatonin every night is probably safe for healthy adults in the short term, with only mild side effects. For long-term use, high doses, children, pregnant women and women who are breastfeeding, there is insufficient evidence to classify this as safe. Furthermore, its effectiveness as a sleep aid for ordinary insomnia is limited.
Short-term use of melatonin in adults has been fairly well studied and found to be safe. Reported side effects are mild: think dizziness, headache, nausea and daytime sleepiness. These side effects are comparable to what people report with a placebo. Serious adverse effects have not been identified in the available randomised studies (PMID 26692007).
For long-term daily use in adults the picture is similar: even over the longer term, side effects are mild and comparable to placebo. Nevertheless, it is important to recognise that the volume of long-term data is more limited than for short-term use. At high doses or in specific groups (children, pregnant women) there is insufficient evidence to offer reassurance (PMID 26692007, 36235587).
Pregnant women and women who are breastfeeding are explicitly advised against using melatonin. There are simply too few human safety studies available to properly assess the risk to mother and child (PMID 26692007). The same applies to children and adolescents outside specific medical indications: long-term safety in this group has not been sufficiently studied.
A particular point of concern is that melatonin is an unregulated supplement in many countries. This means that the actual dosage can differ from what is stated on the label. In older adults, residual daytime sedation is also a real risk, even at low doses (PMID 27751669).
Regarding effectiveness as a sleep aid for insomnia: melatonin scores considerably worse than common sleep medications such as benzodiazepines, zolpidem or orexin receptor antagonists in large network meta-analyses. It has some effect on sleep onset latency (falling asleep slightly faster), but the clinical benefit in chronic insomnia in adults is limited (PMID 35843245, 36701954). An exception is its use in children with autism spectrum disorder: extended-release melatonin showed significant improvements in sleep duration and sleep onset latency after 52 weeks, with the main side effects being fatigue (5.3%) and mood swings (3.2%) (PMID 30132686).
All claims are based on one primary review (PMID 26692007) and two network meta-analyses (PMID 35843245, 36701954), supplemented by a study in older adults (PMID 27751669), an RCT in children with autism (PMID 30132686) and a review with a commercial conflict of interest (PMID 36235587). The evidence base for long-term safety is moderate to limited.