There are various different forms of vitamin D supplements. I found this confusing. I bought calcitriol over the counter thinking it is the most direct form of vitamin D with my limited knowledge and confused state about this vitamin. I didn't use it for a long time because I still was not sure to take this supplement; I had other health problems to deal with and was on so many other medications. A month after the purchase it was sunny and I opt for natural sunshine. In December my blood test show I was a little low on vitamin D, just over the limit. My previous test in July showed my vitamin D level was perfect. I can't remember the exact values, this time I didn't bring my test results with me. But I remember this pattern last year too. I feel I need to supplement in winter months. Now I'm in the UK, sunlight is very limited in January so, I started using this calcitriol. However, I'm kind of worried about it as I found out few articles mentioning calcitriol as a hormone, it is used treating parathyroid problems and supplementing vitamin D for kidney patients. It is not sold here in the UK over the counter. I guess I need to see my GP about this. She mentioned I need to supplement, vitamin D is so important she said. I had 5 doses of this and I think it is fare to say that it helped in many ways, especially the flank pain is improved and a better mood, little energy boost. The dose of it quite high so I take it every other day. Any thoughts?