iodine. applications of tincture of iodine or betadine can cause rashes, sometimes severe. parenteral use of iodine-based contrast agents (see above) can cause reactions ranging from a mild rash to fatal anaphylaxis. such reactions have led to the misconception (widely held, even among physicians) that some people are allergic to iodine itself; even allergies to iodine-rich seafood have been so construed. in fact, there has never been a confirmed report of a true iodine allergy, and an allergy to elemental iodine or simple iodide salts is theoretically impossible. hypersensitivity reactions to products and foods containing iodine are apparently related to their other molecular components; thus, a person who has demonstrated an allergy to one food or product containing iodine may not have an allergic reaction to another. patients with various food allergies (shellfish, egg, milk, etc.) do not have an increased risk for a contrast medium hypersensitivity. as with all medications, the patient's allergy history should be questioned and consulted before any containing iodine are administered. us dea list i status phosphorus can reduce elemental iodine to hydroiodic acid, which is a reagent effective for reducing ephedrine or pseudoephedrine to methamphetamine. for this reason, iodine was designated by the united states drug enforcement administration as a list i precursor chemical under 21 cfr 1310.02.