Ferrous sulfate is a mineral. It supplies iron to your body, which is needed to help your blood carry oxygen to your organs. The medicine is often used to help people with iron-deficiency anemia, whose bodies have too few red blood cells because of poor diet, excess bleeding, other disorders, or pregnancy.ommon Side Effects of Ferrous Sulfate. Ferrous sulfate may cause constipation and upset stomach. You should tell your doctor if these side effects become severe or do not go away. The medicine may also turn your stools dark, which is harmless.