Common side effects of iron supplements include diarrhea and constipation (the latter might necessitate taking a stool softener), as well as passing black stools, which is actually seen as a sign the iron supplements are working. However, if the stools are tarry or have red streaks, or if you suffer cramps, soreness or sharp pains in the stomach, you should contact your doctor immediately. In addition, taking liquid iron supplements may stain your teeth, which you can remove by brushing with baking soda or peroxide.
An overdose of iron can lead to side effects such as vomiting (including vomiting blood), diarrhea and liver failure, which can progress to shock and/or impaired consciousness. Iron toxicity associated with organ failure in pregnant women can lead to spontaneous abortion, preterm delivery and maternal death.
Excess iron has also been linked to neurodegenerative diseases such as Alzheimer’s and Parkinson’s, and may increase the risk of cancer.