Vitamin D is a fat-soluble vitamin. Very few foods naturally contain vitamin D.
Very few foods naturally contain vitamin D (fatty fish livers are the exception), so dermal synthesis is the major natural source of the vitamin. Vitamin D from the diet or dermal synthesis is biologically inactive and requires enzymatic conversion to active metabolites.
Vitamin D is converted enzymatically in the liver to 25-hydroxyvitamin D (25[OH]D), the major circulating form of vitamin D, and then in the kidney to 1,25-dihydroxyvitamin D, the active form of vitamin D.
For all drugs one need to see the frequency and they are prescribed accordingly. I can understand your anxiety on monthly dose.
well note that Half-life elimination of Vitamin D (Calcifediol): Healthy adults: ~11 days; Stage 3 and 4 CKD: ~25 days.
hence good to go. Thanks for asking and for patent read and concern for your mother.
Wish this helps.