I didn't find any literature citing any ill-effects of long-term supplementation. Only good stuff, he shoudl be alright. As well, his immune system should be in amazing shape given Melatonins ability to improve immune cells
Melatonin as antioxidant, geroprotector and anticarcinogen.
Anisimov VN, Popovich IG, Zabezhinski MA, Anisimov SV, Vesnushkin GM, Vinogradova IA.
Department of Carcinogenesis and Oncogerontology, N.N. Petrov Research Institute of Oncology, Pesochny-2, St. Petersburg 197758, Russia.
The effect of the pineal indole hormone melatonin on the life span of mice, rats and fruit flies has been studied using various approaches. It has been observed that in female CBA, SHR, SAM and transgenic HER-2/neu mice long-term administration of melatonin was followed by an increase in the mean life span. In rats, melatonin treatment increased survival of male and female rats. In D. melanogaster, supplementation of melatonin to nutrient medium during developmental stages produced contradictory results, but and increase in the longevity of fruit flies has been observed when melatonin was added to food throughout the life span. In mice and rats, melatonin is a potent antioxidant both in vitro and in vivo. Melatonin alone turned out neither toxic nor mutagenic in the Ames test and revealed clastogenic activity at high concentration in the COMET assay. Melatonin has inhibited mutagenesis and clastogenic effect of a number of indirect chemical mutagens. Melatonin inhibits the development of spontaneous and 7-12-dimethlbenz(a)anthracene (DMBA)- or N-nitrosomethylurea-induced mammary carcinogenesis in rodents; colon carcinogenesis induced by 1,2-dimethylhydrazine in rats, N-diethylnitrosamine-induced hepatocarcinogenesis in rats, DMBA-induced carcinogenesis of the uterine cervix and vagina in mice; benzo(a)pyrene-induced soft tissue carcinogenesis and lung carcinogenesis induced by urethan in mice. To identify molecular events regulated by melatonin, gene expression profiles were studied in the heart and brain of melatonin-treated CBA mice using cDNA gene expression arrays (15,247 and 16,897 cDNA clone sets, respectively). It was shown that genes controlling the cell cycle, cell/organism defense, protein expression and transport are the primary effectors for melatonin. Melatonin also increased the expression of some mitochondrial genes (16S, cytochrome c oxidases 1 and 3 (COX1 and COX3), and NADH dehydrogenases 1 and 4 (ND1 and ND4)), which agrees with its ability to inhibit free radical processes. Of great interest is the effect of melatonin upon the expression of a large number of genes related to calcium exchange, such as Cul5, Dcamkl1 and Kcnn4; a significant effect of melatonin on the expression of some oncogenesis-related genes was also detected. Thus, we believe that melatonin may be used for the prevention of premature aging and carcinogenesis.
PMID: 16678784 [PubMed - as supplied by publisher]
Melatonin and its biological significance]
[Article in Polish]
Boguszewska A, Pasternak K.
Zaklad Chemii Ogolnej AM w Lublinie.
Melatonin is a hormone produced in human by the pineal body, the endocrine gland localized in the central part of cerebrum. It regulates many vital processes. Its main and best known effect is restoring the natural cycle of organism functions. It is safe and non-addictive sleep-inducing drug, which can eliminate disruptions in our circadian rhythm, in such situations as shift working, changing of time zones (during intercontinental air travelling) or insomnia. It improves mood and quality of sleep. Melatonin function consisting in stabilization of biological rhythms, free radical scavenging or immune system stimulating can delay aging processes. Its appropriate supplementation can prolong life even by decades, keeping our body in good both physical and psychological condition. Additionally, profitable for health properties of melatonin include ability to control some illnesses (prophylaxis of cardiovascular system diseases, neoplastic diseases and other functional disorders of organisms). It makes the immune system stronger, decreases susceptibility of the organism to stress, and improves mood and general feeling.