I've had real trouble with vitamin D supplementation. I was less than 4. Have osteopenia. I had megadoses of vitamin to try to correct this problem. I felt great for the first month or so. Then I started to have INCREASED FATIGUE, body aches, racing heart, etc. I stopped the D out of fear. Felt better but not good. Stopped and started again to test my reaction. Definite bad reaction to D. On an incidental blood test my magnesium was very low. In my search, I found out that D megadoses deplete magnesium. You must have adequate magnesium to process D and calcium correctly. That explains, (I think why I felt great at first but then bad (after magnesium was depleted. ) If you are put on megadoses of D without complementary MAGNESIUM you will have problems. So, possibly later after my parathyroid testing is done) I will be using spray-on magnesium chloride to try to bring levels up again before trying D once more. IMPORTANT - ANYONE WITH SUSPECTED HYPERPARATHYROID DISEASE SHOULD NOT TAKE D SUPPLEMENTS UNTIL CAREFULLY EVALUATED BY A DOCTOR. D will make calcium levels go to dangerous levels if you have this disease. GO TO PARATHYROID.COM - I am currently being tested for this issue so will not be doing any d until I know the answer to this. Have a nodule that looks like parathyroid adenoma but not sure. So, if you have problems with D supplementation, definitely explore your PTH, calcium (ionized and urine) magnesium. ALSO if you have any kind of immune illness you must be careful with D. Hope this helps. I am still learning myself so these are just issues to consider. Anyone in my position please write. Will be wonderful to hear from you!