Iron may cause stomach and intestinal disturbances, however, and some experts believe that low doses of ferrous sulfate can be taken with food and absorbed without side effects. One should always drink a full eight ounces of fluid with an iron pill. Common side effects of iron supplements include constipation and diarrhea, but they are rarely severe. Nausea and vomiting may occur with high doses, but can be controlled by taking smaller amounts. Switching to ferrous gluconate may help some people with severe gastrointestinal problems.
Black stools are normal when taking iron tablets.
In fact, if they do not turn black, a physician should be called because the tablets may not be working effectively. This is especially a problem with coated or long-acting iron tablets. If the stools are tarry looking as well as black, if they have red streaks, or if cramps, sharp pains, or soreness in the stomach occur, gastrointestinal bleeding may be causing the iron deficiency and the patient should call the physician promptly. Acute iron poisoning is rare in adults but can be fatal in children who take adult-strength tablets. Iron tablets can aggravate existing gastrointestinal problems such as ulcers and ulcerative colitis.