I began taking vitamin D a few years ago when I was diagnosed with copd. It is one of the vitamins that help with lung health. I see a hematologist yearly and he checked my D levels along with my yearly blood work, found I was low and told me how much D to take to raise my levels to normal. I take 4,ooo iu per day I believe and take it with a meal. I also try to get outside in the sunlight during the spring and summer for a least 30 minutes during the hours between 11-2:00. I buy it over the counter but my husband has a prescription for D prescribed by one of his doctors. He takes one high dose pill a week. I take my gel caps daily. I heard Dr. Fauchi say in an interview that he personally takes Vitamin D and C. I have not been taking extra C but may add it. Some vitamins might interfere with any other medications you are taking so you might need to check to make sure you have no interactions. If you are wondering about your levels, on your next doctor visit ask to get your levels checked. Taking the 4,000iu ( 2 gel caps of 2,000each)is keeping me in the correct levels now.