Prior to taking this supplement, 8 years ago my annual physical revealed that I had a low vitamin D level. The doctor gave me a prescription I had to take for 12 weeks to increase my vitamin D level to a normal level. In my research I found that vitamin D is important as it works with calcium to improve bone strength. At the time, testing revealed that I was also starting to develop Osteoporosis. Coincidentally, we were also taking my 92 year old mother to a doctor who prescribe a vitamin D supplement for her that provided over 10,000 IU of vitamin D. After a couple of months she got sick and, with more research, her symptoms seemed to indicate vitamin D toxicity. The next day, we had her tested and her doctor said he had never seen a vitamin D toxicity level so high. It took over 3 months for the vitamin D to work out of her system, and for her to feel healthy again. So, naturally, I was hesitant in choosing a vitamin D strength for me to take. I know there are much stronger doses of this vitamin that some people take, but having seen the results of toxicity in my mother, I wouldn't risk a high dose. I have been taking this supplement for over 8 years, and my vitamin D levels are at a normal level when I have the annual blood tests done. In addition, with this supplement and the bone supplement I take, my bone density tests have improved over the last 6 years. Since I am now 75 years old, this isn't too shabby. I would definitely recommend this vitamin D supplement, and I hope this review was helpful for you