Hi nyuszisue

I take 1 tablet per month of Vit D3 as prescribed for me by my GP. I have no constipation problems (many years). I've never had it confirmed but I'm guessing the body can store Vit. D - hence the once a month tab.
The product details are: Brand name: Cal.D.Forte
Contains: 1.25mg Colecalciferol BP, Equivalent 50,000 I.U. Vitamin D3

Reading the fine print I see the product is made in NZ (my country) but we have a govt. bulk pharmaceutical supply system here. These details should be enough to ask your PCP if it's available in US.
Hope this helps.