If you are iron-deficient or have anemia, you should talk to your doctor before using ferrous fumarate together with omeprazole. By reducing stomach acid, omeprazole may reduce the absorption of iron and make ferrous fumarate less effective in treating your condition. Your doctor or pharmacist may be able to offer suggestions on safer alternatives if you require treatment for stomach acid or ulcer while you are being treated with ferrous fumarate. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.