Vitamin D is involved in the regulation of CYP3A and CYP2B6 enzyme expression.[16,17] However, CYP3A catalyzes the 4-hydroxylation of 25 (OH)D3, and hence CYP3A induction may contribute to drug-induced vitamin D deficiency.[18,19] Long-term CYP2B6 and CYP3A induction by efavirenz is influenced by pharmacogenetic factors relevant for efavirenz disposition.[10,15,20–22] Accordingly, host genetic factors affecting the plasma concentration of CYP3A inducers may also potentially affect the vitamin D levels in patients on long-term treatment with efavirenz or rifampicin. Here, CYP3A4 is linked to vitamin D deficiency.