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.
Certain medications, including antacids, can reduce iron absorption. Iron tablets may also reduce the effectiveness of other drugs, including the antibiotics tetracycline, penicillamine, and ciprofloxacin and the anti-Parkinson's Disease drugs methyldopa, levodopa, and carbidopa. At least two hours should elapse between doses of these drugs and iron supplements. In some cases iron can be administered through injections, but side effects can be very severe and include blood clots, fever, joint aches, headache, and rashes. Very rarely, fatal allergic reactions have occurred as the result of iron injections.