CYP3A4 and vitamin D deficiency: Vitamin D is involved in the regulation of CYP3A enzyme expression.[18,19] CYP3A4 enzyme activity and expression are induced by 1,25-(OH)D3 via the vitamin D receptor.[44] In addition, CYP3A catalyzes the 4-hydroxylation of 25 (OH)D3 and hence CYP3A induction may contribute to drug-induced vitamin D deficiency.[18,19]