Been taking this for 2.5 weeks for my super low vit D level. The first week I took 1 per day, then bumped up to 2 per day. After 3 days of that, the lowest part of my back started hurting pretty bad. Didn't connect it with the D. Continued 2 more days, then put 2 and 2 together and researched it. Turns out this is common. It's the D shuttling calcium (along with water) into my bones, so my poor bones are sort-of swelling to accommodate. I've never had Back pain in my life. I've been taking this with K2. Upon further reading, you should take lots of magnesium Glycinate with this and the K2. I'm reading that this Back pain can take 3-6 months to resolve. ðŸ˜