You should be aware that iron can turn bowel movements dark green or black, and may cause diarrhea or constipation. If constipation develops, try to eat more high-fiber foods such as beans, vegetables, and whole grain breads and cereals. Keep iron away from children. An overdose can be poisonous. The body absorbs only a small amount of the iron in food.
Red meat is the best source.
Any more than this will keep the iron from being absorbed by your body. Other foods that prevent absorption are coffee, tea, egg yolks, and yogurt. You develop heartburn, constipation, diarrhea, nausea, or vomiting. You feel weak or dizzy. You have a fever.