Ferrous sulfate
Ferrous sulfate is an oral iron salt sold over the counter. The iron it supplies is absorbed in the upper small intestine and used to build haemoglobin and to refill body iron stores. It is taken for iron deficiency, with or without anaemia.
Absorption is best on an empty stomach and is helped by vitamin C, while tea, coffee, and calcium-containing supplements bind iron in the gut and blunt it. Iron also binds several medicines and reduces their absorption, so levothyroxine, doxycycline and ciprofloxacin need separating in time from it, and omeprazole lowers the stomach acid iron needs; it competes with zinc as well. Dark stool is expected and harmless, constipation is common, and ferritin, haemoglobin, serum iron and transferrin saturation are the values used to judge the response.
See a doctor if abdominal pain, vomiting, or black tarry stool with lightheadedness occurs, since that suggests bleeding rather than the harmless darkening, and if fatigue or blood counts do not improve, since unexplained iron deficiency needs its cause found.
Reference information, not medical advice. Discuss any change with your doctor.