The Schilling test has multiple stages. As noted below, it can be done at any time after vitamin B supplementation and body store replacement, and some clinicians recommend that in severe deficiency cases, at least several weeks of vitamin repletion be done before the test (more than one B shot, and also oral folic acid), in order to ensure that impaired absorption of B (with or without intrinsic factor) is not occurring due to damage to the intestinal mucosa from the B and folate deficiency themselves.