The effect of medications is a major problem since a patient may be taking several drugs or may be taking over-the-counter pharmaceuticals without reporting them to the physician. Medication effects may be manifest in several ways: drug-induced injury to tissues or organs (e.g., isoniazid-induced hepatitis), drug-induced alterations in organ function (e.g., increase in g-glutamyltransferase produced by phenytoin microsomal induction in liver cells), drug competition effect (e.g., displacement of thyroxine from thyroxine-binding proteins by phenytoin), and interference by one drug with theanalysis method of another (e.g., decrease in serum glucose using glucose oxidase when large doses of vitamin C are ingested).