The Schilling test has multiple stages.[3] As noted below, it can be done at any time after vitamin B12 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 B12 shot, and also oral folic acid), in order to ensure that impaired absorption of B12 (with or without intrinsic factor) is not occurring due to damage to the intestinal mucosa from the conditions of malabsorption arising from B12 and folate deficiency themselves.