I have to look this up myself every time:
One study, albeit small, looked at the effects of vitamin D3 versus D2 supplementation and found that D2 potency is less than 30% of that of D3 and that it has a markedly shorter duration of action. Because the 1,25 form is metabolized in the kidney, D2 is not recommended for patients with CKD or ESRD (D3 should be used). One paper even argues that vitamin D2 should not be sold as a supplement anymore.
Random fact: vitamin D supplementation is a USPSTF grade B recommendation for elderly adults for fall prevention.
Prescribing vitamin D for vitamin D-deficient patients is surprisingly controversial. There are gray areas like what truly counts as “low vitamin D,” racial differences in vitamin D levels (most discussions of vitamin D supplementation are based on evidence in Caucasians), and who should be screened in the first place.
However, here are the quick and dirty guidelines from UpToDate:
Generally, vitamin D deficiency is a serum 25 (OH)D level <20 ng/ml. A couple of specialty societies suggest that a level <30 ng/ml is cause for supplementation in pts >age 65. However people are usually not at risk for osteomalacia unless <10 ng/ml.
Normal adults do NOT need to be screened, but the elderly, those with poor sunlight exposure and malabsorptive disease, should be.
D3 (cholecalciferol) is thought to be more efficacious than D2 (ergocalciferol). Although you will often see someone prescribed 50,000 U weekly followed by 600-800 U daily, there is no evidence behind the 50,000–so you might as well just start them on 600-800 U daily. Vitamin D levels should be monitored every 3-4 months until the target level is met. If someone has malabsorptive disease or isn’t responding to initial treatment, they may require increase of their dose.