 [ Symptoms ] i was taking 300 mcg of melatonin now i can only find 3 mg which is stronger [ Disease ] one milligram mg is equal to 1 000 micrograms mcg therefore if you were taking 300 micrograms of melatonin then you would need to find a product that is 0 3 milligrams of melatonin the highest amount of melatonin found over the counter is 3 milligrams or 3 000 micrograms there are different strengths available if you cannot find the dose you are looking for speak with your local pharmacist they can order different strengths for you to purchase that are still over the counter and that do not require a prescription