As a hormone produced naturally by our bodies, it’s easy to dismiss the possibility of a melatonin overdose. But even though the word “overdose” conjures up very different and dramatically more severe connotations, consuming too much melatonin is technically possible and can carry some negative implications.
While melatonin’s side-effects are generally relatively mild and melatonin intake is easy to adjust on the fly, some awareness and caution can help you avoid those side-effects but get the most out of this supplement. Moreover, research on the long-term effects of melatonin is very scarce, which is another reason to play it safe just in case.
In many ways, consuming melatonin is reminiscent of CBD.
Side-Effects of Melatonin Overconsumption
Anecdotal evidence suggests the most common side-effects of melatonin overconsumption are headaches, grogginess, dizziness, and nausea – pretty much the same as CBD. 
“In general, animal and human studies documented that short-term use of melatonin is safe, even in extreme doses,” a study in the Clinical Drug Investigation journal states. “Long-term safety of melatonin in children and adolescents, however, requires further investigation.” 
Does it Disrupt Natural Melatonin Production?
A universal concern regarding regular intake of exogenous hormones that occur naturally in the body is if would affect the body’s ability to produce the said hormones. This was practically ruled out by a study, which explored two scenarios, be it short-term. 
In the first, 0.5 mg was taken daily for seven days. Endogenous melatonin production was the same on the 8th day. The second scenario was far more drastic and involved a blind person, as visual impairment hinders light perception, disrupting the circadian rhythm. The blind subject was given a 50mg daily bedtime dose – the recommended dose is between 1 and 10mg – over 37 days. Still, there was “no change in endogenous melatonin profile.”
Despite blind people generally require larger melatonin dosages, 50mg of 37 days is still a lot, and these results should still serve as an indicator for melatonin’s relative safety.
Tolerance and Withdrawal Symptoms
Building up tolerance and withdrawal symptoms is another universal concern when taking exogenous substances and medications.
So far, melatonin has appeared to be very non-intrusive in that regard, too. In a study that involved 3-week and 6-month trial periods, with participants aged 55 or over, short- or longer-term treatment with prolonged melatonin release 2 mg was not associated with dependence, tolerance, rebound insomnia or withdrawal symptoms.” 
Other studies come to pretty much the same conclusions – that melatonin is generally entirely benevolent and safe.
With that being said, bear in mind the 2mg dose cited above is quickly exceeded, and these studies still explore relatively short-term circumstances. For that, it’s best to err on the side of (moderate) caution and if you’re going to increase intake, do so very gradually.
- Andersen LP et al, The Safety of Melatonin in Humans, Clin Drug Investig.2016 Mar;36(3):169-75, Impact Factor = 2.267
- Matsumoto et al, The amplitude of endogenous melatonin production is not affected by melatonin treatment in humans, J Pineal Res 1997 Jan;22(1):42-4; Impact Factor = 15.2; Times Cited = 2
- Katherine A Lyseng-Williamson, Melatonin prolonged release: in the treatment of insomnia in patients aged ≥55 years, Drugs Aging 2012 Nov;29(11):911-23, Impact Factor = 2.381