Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. Melatonin usually safe but higher doses it may cause side effects like depression, bleeding disorders aggravation, diabetes aggravation, seizure disorder, transplant recipients may interfere with immunosuppressive therapy. I think you also have irritable bowel syndrome or malobsorption syndrome lead to vitamin deficiencies. You need to get it done stool examination and endoscopy. Please consult your gastroentrologist he will examine and treat you accordingly. Hope I have answered your query. Let me know if I can assist you further.