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Melatonin (5-Methoxy-N-acetyltryptamine, CAS Reg. No. 73-31-4) is a neurohormone that is used for medicinal purposes, primarily for treatment of  sleep-related disorders. Various credible sources and reports in the scientific literature, some of which we will refer to below, describe information on melatonin that would raise safety concerns about its use as a food ingredient.

In a study published in 2001 (Anisimov et al., Journal of Gerontology: Biological Sciences, 56A:B311-B323, 2001), melatonin was reported to increase spontaneous malignant tumor incidence when administered orally to female CBA mice, although it increased the life span of these animals. Based on this observation, the authors advised caution regarding use of melatonin as an anti-aging drug.

A recently published study (Wiechmann et al., Experimental Eye Research, 86:241-250, 2008) shows melatonin, administered orally, to be toxic to retinal photoreceptor cells of male and female Fischer 344 (non-pigmented) rats and male pigmented LongeEvans rats, depending on the conditions of exposure to light. In their conclusion, the authors cautioned that chronic exposure to natural or artificial light and simultaneous intake of melatonin may potentially contribute to a significant loss of photoreceptor cells in the aging retina.

Melatonin is the subject of a monograph prepared by the Natural Standard,2 an international research collaboration that gathers and reviews data on complementary and alternative therapies. The safety assessment that the monograph provides is with respect to the medicinal use of melatonin. The monograph states that, based on available studies and clinical use, melatonin is generally regarded as safe at recommended doses for short-term use only and describes various safety concerns about its use, some of which we mention here. The monograph states that melatonin may cause drops in blood pressure, as observed in animals and in preliminary human research and caution is advised in patients already on anti-hypertensive medications. The monograph states that preliminary research suggests that regular use of melatonin may increase atherosclerotic plaque buildup in humans and animals. The monograph states that hyperglycemia has been reported in patients with type-1 diabetes, and low doses of melatonin have reduced glucose tolerance and insulin sensitivity. The monograph states that gynecomastia has been reported in men, as well as decreased sperm count (both of which resolved with cessation of melatonin). It also states that decreased sperm motility has been reported in rats and humans. The monograph states that there are limited studies regarding use of melatonin in children (younger than 18 years) and safety has not been established. It further states that melatonin should not be used in children without consultation with a physician. The monograph states that melatonin use should be avoided in women who are pregnant or attempting to become pregnant, based on possible hormonal effects, including alterations of pituitary-ovarian function and potential inhibition of ovulation or uterine contractions. It also states that high levels of melatonin during pregnancy may increase the risk of developmental disorders.