Hi Lora: D3 (cholecalciferol) is the form you get from the sun. Your skin makes D3 when it's exposed to the sun for 15 minutes without sunscreen.  If you are trying to get D3 from the sun, you would have to expose your skin several times a week and live in a sunny climate.  As we age getting enough D3 from the sun is very difficult so supplemental D3 is used for those who have difficulty synthesizing D3 from the sun.  Fish liver oils, and eggs also provide vitamin D.

D2 (ergocalciferol) is the form used in fortified foods, like milk, juice, cereal and supplements.

D3 is recommended by some over D2, at 1,000 International Units a day, for those over the age of 50.  When taking vitamin D, you would choose either D2 or D3 and then the required amount, you wouldn't take both D2 and D3, just one or the other.

If you haven't had your vitamin D levels checked, you may want to ask your Dr to run a test to see how much you have stored in your body.  The test is a simple blood test and most insurances cover it.  The name of the test is simply vitamin D test or the medical term is 25-hydroxyvitamin D test.

With these test results your Dr would be able to tell you how much you should take.  If you are extremely deficient in D you *may* have to go on a higher dose than recommended above.

Good luck