Dr. Ashely Alker, an emergency medicine physician in Fredericksburg, Virginia, has highlighted the dangers of iron, a mineral that is a common ingredient in some multivitamins. Iron can be harmful in large quantities and is particularly dangerous for children. Most people who take iron get it in a multivitamin or in women's prenatal vitamins, which can be in the form of gummies. An overdose of iron supplements can be life-threatening, especially in children. High doses of iron can cause an upset stomach, constipation, nausea, abdominal pain, vomiting and diarrhea. Alker recommends locking up any adult supplements with iron in them out of sight and out of reach.