Doctors only check the blood for magnesium. Problem is the blood only has 1% of the total magnesium . There are other tests that tell the cellular levels of magnesium I think it is called a specrasite test or something similar to that. Anyway you need to ask the doctor to test for cellular levels not blood levels. It is my understanding that massive long term amounts are needed. I am trying a new type called jigsaw that is a time release version of magnesium. Can't tell if it will work since I also take 2 other types with no results yet.I know I am low since I seldom and have never had any colon issues with magnesium no matter how much I take.