Is it possible to be allergic to melatonin?

Allergy. Allergic antibodies can be formed to any foreign protein however, melatonin is not a foreign protein. At times, humans develop allergy to self-body proteins such as Insulin but this is very rare. Allergy can also occur to pill binders such as hypomellose or croscarmellose but these reactions are also rare. If a drug repeatedly causes a adverse side effect, it should be avoided.