Corticosteroid drugs (medicines related to cortisone, a substance produced by the adrenal glands) are commonly used to treat patients with inflammatory diseases or diseases of the immune system. Many of these patients also require treatment with anticoagulant drugs (“blood thinners”). Anticoagulants are used to treat or prevent blood clots (for example, in the brain [strokes], legs [thrombophlebitis], or lungs [pulmonary embolism]). Many drugs can interfere with the action of anticoagulants; some cause them to thin the blood too much, others to thin it too little. Corticosteroids have not been thought to interfere with anticoagulants, but few reports on this issue exist. Methylprednisolone is a commonly used intravenous corticosteroid.