As already pointed out, melatonin is a simple amino acid, which is broken down, resorbed and completely used up by the organism a few hours after its administration. It is absolutely non-toxic. Even during more than thirty years of extensive international research, no side effects were observed which could conclusively be traced to the application of melatonin. Don't forget, up to the age of about 45, your own body produces abundant amounts of melatonin without you suffering from it. On the contrary, it helps to provide you with good health and well-being. Thus, there is no reason to assume that it will cause allergic reactions when taking it as a supplement after the age of 45.