Ferrous Sulfate

Overview

Ferrous sulfate is an oral iron supplement used in dogs and cats for the treatment of iron deficiency anemia and as a hematinic agent. It provides a source of elemental iron required for hemoglobin production and oxygen transport.

It is commonly used in cases of chronic blood loss or as adjunctive therapy in patients receiving erythropoiesis-stimulating agents such as epoetin or darbepoetin. However, correction of the underlying cause of iron deficiency is essential for effective treatment.

Mechanism of Action (MOA): Ferrous sulfate supplies bioavailable iron necessary for the synthesis of hemoglobin and myoglobin, and for the electron transport chain, oxidative phosphorylation and other oxidative reactions in metabolism. After absorption, iron is transported via transferrin to the bone marrow where it is incorporated into hemoglobin. It corrects anemia only when iron deficiency is present and does not stimulate erythropoiesis in non–iron-deficiency conditions.

Indications

Ferrous sulfate is used in dogs and cats as an oral iron supplement for the treatment and support of conditions associated with iron deficiency. It is most effective when iron deficiency is confirmed and the underlying cause is addressed.

• Iron deficiency anemia: Used to treat anemia caused by chronic external blood loss – hookworms, gastrointestinal ulceration or neoplasia, severe flea infestation, urinary tract hemorrhage, blood-donor overuse or repeated diagnostic sampling. Inadequate dietary iron, such as an inappropriate home-cooked diet, is a separate cause of iron deficiency.
• Adjunct to erythropoietin therapy: Administered alongside epoetin or darbepoetin when iron supplementation is indicated, to support red blood cell production.
• Nutritional iron supplementation: May be used in patients with inadequate dietary iron intake or increased physiological demand.
• Chronic kidney disease: Iron supplementation may be used in chronic kidney disease. Inflammatory hepcidin can sequester iron and limit gastrointestinal absorption, contributing to functional iron deficiency. Note that iron-salt guidance also lists renal disease, particularly pyelonephritis, among its contraindications – assess the indication, the route and the renal disorder individually.

Dosage (Reference)

Dog

In dogs, ferrous sulfate is used as an oral iron supplement for iron deficiency anemia or in patients receiving erythropoiesis-stimulating agents. Oral iron may be used when feasible, while parenteral iron may be preferred initially in severe iron deficiency or when oral therapy is poorly tolerated, ineffective because of malabsorption or continuing severe blood loss, or impractical because of poor compliance. Oral therapy may be started or continued once adequate iron repletion and gastrointestinal tolerance permit.

Cat

In cats, Oral ferrous sulfate may be used for iron-deficiency anemia, but oral iron is often poorly tolerated; parenteral iron, particularly iron dextran, may therefore be preferred when oral supplementation is not tolerated or adequate absorption cannot be ensured.

Warnings & Precautions

Ferrous sulfate is generally safe when used appropriately, but its gastrointestinal effects and risk of toxicity require careful patient selection, dosing accuracy, and product evaluation.

• Hypersensitivity: Contraindicated in animals with known hypersensitivity to ferrous sulfate or any component of the formulation.
• Iron overload disorders: Use with caution in patients with hemosiderosis or hemochromatosis, as additional iron may worsen tissue accumulation and toxicity.
• Hemolytic anemia: Not indicated in hemolytic anemia, and contraindicated in any anemia that is not due to iron deficiency – iron will not correct it and adds the risk of overload.
• Gastrointestinal disease: Oral iron irritates the gastrointestinal tract. Do not use in animals prone to gastric ulcers, and avoid it where gastrointestinal ulceration is already present.
• Product variability: No FDA-approved veterinary products exist, and quality and composition vary between formulations; look for a third-party verification mark such as USP or NSF on the bottle to confirm the product contains what it claims.
• Formulation components: Some preparations contain alcohol or propylene glycol; check the formulation before use.
• Dosing accuracy (critical): Confusion may occur between doses expressed as ferrous sulfate versus elemental iron—verify dosing carefully to avoid under- or overdosing.
• Use in pregnancy and lactation: Iron requirements increase during pregnancy and oral iron may be used to correct deficiency in pregnant animals; in women ferrous sulfate is considered compatible with breastfeeding.
• Kittens: Avoid oral iron in kittens – it has been associated with unexplained death. Transient iron-deficiency anemia may occur at 5 to 10 weeks of age, and kittens begin spontaneous iron repletion from about 5 to 6 weeks.
• Systemic disease and inflammation: Iron supplementation should be avoided in severe infection or inflammation, hepatic or cardiac disease, active urinary tract infection, and certain renal disorders, particularly pyelonephritis. Renal disease is not an absolute contraindication in all circumstances, as iron supplementation may be indicated in chronic kidney disease with iron deficiency; the underlying renal condition and route of administration should therefore be considered individually.
• Accidental ingestion: Keep ferrous sulfate out of reach of children and other animals; accidental overdoses can be very toxic, and overdoses can be life-threatening.
• When to contact a veterinarian: Tell the owner to contact a veterinarian immediately if black tarry stools or bloody vomit occur.

Drug Interactions

Reported interactions mainly involve reduced absorption of iron or of other oral drugs, through binding or altered gastrointestinal conditions; some are theoretical or are based on human or general animal data. Dose separation is often required.

• Antacids (aluminum or magnesium-containing): May bind iron and reduce its absorption; administer at least 2 hours apart.
• Calcium supplements / antacids: Decrease iron absorption; separate administration by at least 2 hours.
• Fluoroquinolones (e.g., enrofloxacin, marbofloxacin): Iron can bind these drugs and reduce their absorption; give 2–4 hours apart.
• Tetracyclines: Oral iron binds tetracyclines and decreases the absorption of both compounds.
• Levothyroxine: Iron decreases its absorption; administer at least 4 hours apart.
• Penicillamine: Oral iron reduces its absorption; give at least 1 hour apart.
• Mycophenolic acid: May reduce drug efficacy; separate administration by at least 2 hours.
• Chloramphenicol: May delay response to iron therapy; avoid use in patients with iron deficiency anemia.
• H2-receptor antagonists (e.g., famotidine, ranitidine): May decrease iron absorption.
• Proton pump inhibitors (e.g., omeprazole): May reduce iron absorption by altering gastric pH.
• Alendronate: Iron reduces absorption of oral bisphosphonates; administer at least 2 hours apart.
• Vitamin C: May enhance iron absorption.
• Sucralfate: Reduces the absorption of oral iron; separate administration by at least 2 hours, giving the iron first.
• Sulfasalazine: Oral iron may decrease blood concentrations of sulfasalazine; the clinical significance is unknown.

Side Effects & Overdose

Side Effects

Adverse effects of ferrous sulfate in dogs and cats are usually limited to mild gastrointestinal irritation. Most cases are mild and improve with dose adjustment or administration with food.

• Gastrointestinal upset: Includes abdominal discomfort, nausea, vomiting, constipation, or diarrhea.
• Dose-related intolerance: Some animals may require dose reduction or divided dosing to improve tolerance.
• Gastric ulceration at high doses: High doses can cause gastric ulceration, and high doses or accidental ingestion may cause severe ulceration and perforation – treat as an emergency.

Overdose

Iron overdose is a medical emergency and can result in severe, life-threatening toxicity. Clinical signs may occur rapidly after ingestion or may be delayed, progressing through multiple phases of toxicity.

• Toxic dose: In dogs, toxicity has been observed above 20 mg/kg of elemental iron, with mild signs around 20 mg/kg and severe signs around 60 mg/kg. Ingestion of more than 50 mg/kg generally warrants decontamination, and more than 80 mg/kg needs treatment and monitoring beyond it. Doses are calculated as ELEMENTAL iron, so work out how much elemental iron the product ingested contains. Metallic iron and iron oxide (rust) are not readily ionizable or absorbable, and toxicity is not expected from those forms.
• Acute GI irritation: Early signs include vomiting, which may be hemorrhagic, abdominal pain and diarrhea; these may appear within 30 minutes of ingestion but can be delayed for several hours.
• Systemic toxicity: May progress to depression, weak or rapid pulse, hypotension, cyanosis, ataxia, and coma due to circulatory collapse.
• Delayed critical phase: After an initial period of apparent improvement, a second critical phase may occur, typically within 12–48 hours after ingestion, although delayed deterioration has been reported up to 96 hours.
• Severe complications: Pulmonary edema, vasomotor collapse, hepatic failure, coma, and death may occur in advanced cases.
• Long-term sequelae: Survivors may develop gastric scarring or persistent gastrointestinal dysfunction.
• Diagnosis: Based on the history, radiopaque tablets on abdominal radiographs, and serum iron with total iron-binding capacity. Draw the serum iron 4 to 6 hours after ingestion of tablets or capsules and 2 to 3 hours after a liquid or chewable product, repeat every 2 to 4 hours until the peak if possible, and avoid a hemolyzed sample – hemolysis falsely raises the iron result.
• Treatment: Supportive care plus chelation, and chelation is most effective within 24 hours of ingestion, before iron distributes from the blood into the tissues. Chelate when serum iron exceeds 350 to 500 micrograms/dL or exceeds the total iron-binding capacity, using deferoxamine mesylate 40 mg/kg IM every 4 to 8 hours, or a 15 mg/kg/hour intravenous infusion in critical patients. Administer intravenous deferoxamine slowly. Continue until clinical signs resolve, serum iron falls below 300 micrograms/dL and the urine no longer turns reddish-brown after treatment – reported to take 12 to 72 hours. Correct hypovolemic shock with intravenous fluids and severe acidosis with sodium bicarbonate.
• Decontamination: Activated charcoal does NOT bind iron and is of no value. Induce emesis, or lavage if emesis is contraindicated – but avoid gastric lavage if there is hematemesis, due to the increased risk of perforation. Oral milk of magnesia or aluminum hydroxide, 5 to 30 mL by mouth every 8 to 12 hours for 24 hours, precipitates iron in the gut; surgery may be needed if adherent tablets or a bezoar remain.
• Monitoring and disposition: Admit any patient that has taken a toxic dose or is showing signs; home monitoring may be considered only if the patient stays free of signs through the first 8 hours after ingestion. Monitor liver enzymes for up to 24 hours after excess circulating iron is controlled, and watch during recovery for gastrointestinal stricture or obstruction.
• Poison-control consultation: For a known or suspected overdose, consult a 24-hour poison service that provides veterinary-specific guidance.
• Prognosis: Generally good when treatment starts early, and worse once signs of toxicosis are established or after a very large ingestion. In a retrospective series of 61 dogs with iron-EDTA ingestion, 91.8% survived to discharge, although iron toxicosis can still result in cardiovascular collapse and hepatic failure.

Key Notes

Practical clinical points to optimize the safe and effective use of ferrous sulfate in dogs and cats:

• Elemental iron awareness: Check the product’s labelled elemental iron content before dispensing or converting a dose. Regular ferrous sulfate is about 20% elemental iron by weight – a 325 mg tablet supplies about 65 mg and a 250 mg tablet about 50 mg – while the dried (desiccated) salt is about 30%, and 200 mg of it supplies about 65 mg. Other iron salts differ again, and published dose tables report differing equivalents.
• Absorption variability: Oral iron absorption is influenced by diet, existing iron stores, erythropoietic activity, and the dose given.
• Primary absorption site: Iron is mainly absorbed in the duodenum and proximal jejunum.
• Transport mechanism: After absorption, iron binds to transferrin and is delivered to the bone marrow for incorporation into hemoglobin.
• Closed iron cycle: The body conserves iron efficiently, recycling it from red blood cell breakdown with minimal daily loss.
• Delayed hematologic response: Red cell indices may take months to normalize. Effective treatment is associated with an increase in mean cell volume and reticulocyte volume, and the microcytic population falls over about 2 to 3 months.
• Fecal changes: Large oral doses can darken or blacken the feces and cause false-positive guaiac tests for occult blood.
• Monitoring: Baseline and periodic complete blood count with red cell indices, repeated every 1 to 4 weeks at first, together with serum iron and total iron-binding capacity; serum ferritin approximates total body iron stores. Monitor for adverse reactions as well.
• Functional iron deficiency: In inflammatory conditions, hepcidin sequesters iron and limits gastrointestinal absorption. In dogs with severe iron deficiency, intestinal absorption may itself be impaired, making oral therapy of little value until partial repletion has occurred.
• Storage and handling: Store in tight, light-resistant containers; tablets at 15-30 °C and liquids at 20-25 °C. Do not mix ferrous sulfate directly with other drugs, because chelation may occur. Ferrous sulfate exposed to moisture or moist air oxidizes to a brownish-yellow ferric compound that should not be used.