Is there a difference between a compounding dose of vitamin D (e.g. once a month/qtr/year) vs daily supplementation?

There is insufficient evidence to suggest that efficacy is affected by dosage regime. Compounding pharmacy formulations are not controlled for quality and stability. Ostelin products are manufactured under principles of Good Manufacturing Practice (GMP) which means that there are multiple controls in place to safeguard quality of the product and specifically that the product delivers the label quantity to the end of the shelf life.