Iron: Gastrointestinal side effects such as nausea, vomiting, bloating, and upper abdominal discomfort are seen with high iron doses. Other undesirable effects may include either diarrhea or constipation. Vitamin C: Prolonged intake of ascorbic acid (vitamin C) in excess 2 grams per day may lead to nausea, abdominal cramps, diarrhea, and nose bleeds.