Typically, vitamin D supplement dosage is prescribed based on the how low your levels are. Critically low levels call for frequent supplementation whereas monthly dosages are to maintain adequate levels.

But unlike other supplements taken daily, the reason it is prescribed by doctors in intervals is because vitamin D has a long half life ie., it breaks down slowly in the body. Being fat soluble it can be stored in the body for future use as well.

While the effect of Vitamin D supplements take a few months to show any significant change, small changes like better energy levels and better mood can be observed within a couple weeks. That said, basking in morning sunshine is equally good and a great long term habit to support Vitamin D levels.

P.s. This is not medical advice. This post has been written only for better understanding of vitamin D supplementation.