Vitamin D3 (25-OHD3) analyses have increased exponentially and vitamin D deficiency (< 25 nmol/l) is common (15% of patients). The aim of the paper is to discuss reasons for unsuccessful treatment and to question the use of ergocalciferol (vitamin D2). Lack of effect of treatment can be due to: 1) too low dose, 2) incorrect analytical methods when injection treatment (vitamin D2) is used, 3) obesity, 4) seasonal variations, and 5) poor compliance. Treatment is mandatory in order to prevent osteopenia and osteoporosis. Vitamin D3 is more potent than vitamin D2. Injections with vitamin D2 should be replaced by vitamin D3.