I've recently been diagnosed with osteoporosis at age 64. I had been taking a D2 supplement in a mullti along with OptiOmega 3 which has some D3. This had kept my blood level at 51 ng/l in September 2016 (bearing in mind I had been sunbathing as well) which is just within NHS guidelines. A further test was done in February 2017 when I had been supplementing with this 2500 iu D3 over the winter. My level had risen to 66 ng which the doctor was very pleased with. I took it most days. Vitamin D is vital to help protect against osteoporosis and although for prevention lower level supplements are likely OK, for bone building I understand at least 800 iu daily is needed. Don't forget folks, osteoporosis can result from lack of bone building when you're young, and probably did so with me. If you are a vegan, please do make sure that you take an adequate level of this vitamin to help with the absorption of calcium, along with K2 to mobilise it into the bones. In the old days, very low levels of Vit D were thought to be needed, but this information has been revised upwards recently by the NHS as well. I find this supplement good value and as it's stored, Vit D doesn't need to be taken every day, especially in summer if you are getting adequate sun on your skin.