Adults receive an average dose of 0.1–10 milligrams (mg). Melatonin has a 1–2 hour half-life, depending on the formulation. Half-life refers to how long it takes the body to metabolise half of a drug dose. According to some studies, the body takes about 4–5 half-lives to remove medication. OTC melatonin lasts for 4-10 hours, depending on the dosage and formulation.