Gastrointestinal side effects of taking iron ferrous sulfate are anorexia, heartburn, constipation and diarrhea, notes . These side effects are often dose-related, and they may reduce when doctors prescribe smaller doses and increase the dose gradually. Liquid preparation of iron ferrous sulfate may stain the teeth. To prevent this side effect, patients can dilute the liquid iron ferrous sulfate in water or juice and take it using a straw. Local side effects include gangrene, while metabolic side effects include decreased thyroxine absorption. A rare side effect is secondary hemochromatosis, or iron overload.