Taking too much serotonin by combining medications such as antidepressants, migraine medications and melatonin can lead to a serious drug reaction.

'We cannot be certain of the purity of melatonin that is available over the counter,' Robbins said. Previous studies also found 26% of the melatonin supplements contained serotonin, 'a hormone that can have harmful effects even at relatively low levels,' according to the National Center for Complementary and Integrative Health. Nor are there any requirements that companies test their products for harmful hidden additives in melatonin supplements sold in stores and online. 'Previous research has found that that melatonin content in these unregulated, commercially available melatonin supplements ranged from - 83% to +478% of the labeled content,' said Robbins, who coauthored the book 'Sleep for Success! Everything You Must Know About Sleep But are Too Tired to Ask.' Unlike drugs and food, melatonin is not fully regulated by the US Food and Drug Administration, so there are no federal requirements that companies test pills to ensure they contain the amount of advertised melatonin. However, pills for sale may contain levels of melatonin much higher than what is advertised on the label. Since 2006, a small but growing subset of adults are taking amounts of melatonin that far exceed the dosage of 5 milligrams a day typically used as a short-term treatment, the study found. However, researchers could not determine which type of sleep aid - over-the-counter medications, such as melatonin, or prescription medications - was responsible for the findings. 'In an associational study we found that older adults who reported frequent use - every night or most nights - of a sleep aid (over the counter or prescription) had a higher risk of incident dementia and early mortality,' Robbins said. While short-term use for people with jet lag, shift workers and those who have trouble falling asleep appears to be safe, long-term safety is unknown, according to the National Center for Complementary and Integrative Health at the National Institutes of Health.

It can also interact with common medications and trigger allergies. Melatonin has been linked to headaches, dizziness, nausea, stomach cramps, drowsiness, confusion or disorientation, irritability and mild anxiety, depression and tremors as well as abnormally low blood pressure. The study, published in the medical journal JAMA, found that by 2018 Americans were taking more than twice the amount of melatonin than a decade earlier. The overall use among the US adult population is still 'relatively low,' but the study does 'document a significant many-fold increase in melatonin use in the past few years,' said sleep specialist Rebecca Robbins, an instructor in the division of sleep medicine for Harvard Medical School.

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