Metformin Hydrochloride

Biguanide Antihyperglycemic agent

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 10-20 mg/kg q12h Duration: Long-term; adjust based on glycemic response
Notes: Start at low end and titrate gradually. Administer with food to reduce GI upset. Monitor blood glucose and clinical signs.
Cat PO 10-20 mg/kg q12h Duration: Long-term; adjust based on glycemic response
Notes: Start at 10 mg/kg q12h, increase to 20 mg/kg if needed. Administer with food. Monitor for GI side effects.
Horse PO 15-30 mg/kg q12h Duration: Long-term; adjust based on insulin and glucose levels
Notes: Use with caution; limited evidence. Monitor for GI upset and lactic acidosis.
Cattle PO 10-20 mg/kg q12h Duration: Variable; experimental
Notes: Not approved; use only under veterinary supervision. Monitor for adverse effects.
Small Ruminants PO 10-20 mg/kg q12h Duration: Variable; experimental
Notes: Not approved; use only under veterinary supervision.
Rabbit PO 10-20 mg/kg q12h Duration: Variable; experimental
Notes: Limited data; use with caution.
Bird/Poultry PO 10-20 mg/kg q12h Duration: Variable; experimental
Notes: Limited data; use with caution.
Exotic/Other PO 10-20 mg/kg q12h Duration: Variable; experimental
Notes: Dose based on extrapolation; use with caution.

Clinical Indications & Species Uses

General Indications
  • Diabetes mellitus (type 2)
  • Insulin resistance
  • Polycystic ovary syndrome (in humans; off-label in animals)
Dog (Canine)
  • Adjunct to insulin therapy for diabetes mellitus
  • Type 2 diabetes mellitus (rare in dogs)
  • Insulin resistance (off-label)
Cat (Feline)
  • Adjunct to insulin therapy for diabetes mellitus
  • Type 2 diabetes mellitus (common in cats)
  • Insulin resistance (off-label)
Horse (Equine)
  • Equine metabolic syndrome (insulin resistance)
  • Laminitis associated with hyperinsulinemia (off-label)
Cattle (Bovine)
  • May be used off-label for insulin resistance in dairy cows (experimental)
Small Ruminants (Sheep / Goat)
  • May be used off-label for insulin resistance (experimental)
Rabbit & Small Mammals
  • May be used off-label for diabetes mellitus (rare)
Avian & Poultry
  • May be used off-label for diabetes mellitus (rare)
Exotic & Other Species
  • May be used off-label for insulin resistance in some exotic species (e.g., primates, rodents)

Pharmacology & Mechanism of Action

Drug Class: Biguanide | Pharmacological Group: Antihyperglycemic agent

Mechanism of Action: Metformin is a biguanide antihyperglycemic agent that primarily acts by decreasing hepatic glucose production (gluconeogenesis) and reducing intestinal absorption of glucose. It also improves insulin sensitivity by increasing peripheral glucose uptake and utilization, particularly in skeletal muscle and adipose tissue. The exact molecular mechanism involves activation of AMP-activated protein kinase (AMPK), which leads to inhibition of gluconeogenic enzymes and stimulation of glucose transport. Unlike sulfonylureas, metformin does not stimulate insulin secretion, thus reducing the risk of hypoglycemia.

Pharmacodynamics: Metformin lowers both basal and postprandial plasma glucose concentrations. It improves glycemic control by reducing hepatic glucose output and enhancing peripheral insulin sensitivity. It also has beneficial effects on lipid metabolism, reducing triglycerides and LDL cholesterol, and may promote weight loss or prevent weight gain. In veterinary patients, its effects are less pronounced than in humans, and it is often used as an adjunct to insulin therapy or in early-stage diabetes mellitus.

⚡ Pharmacokinetics Summary

Absorption: Metformin is absorbed from the small intestine with an oral bioavailability of approximately 50-60% in humans. In dogs, absorption is variable and may be lower; food decreases the rate and extent of absorption. Peak plasma concentrations occur within 2-3 hours after oral administration.
Distribution: Metformin has a low volume of distribution and does not bind significantly to plasma proteins. It distributes into erythrocytes and accumulates in the gastrointestinal tract, liver, and kidneys. It crosses the placenta and is excreted in milk in small amounts.
Metabolism: Metformin is not metabolized in the liver; it is excreted unchanged in the urine. It undergoes minimal hepatic metabolism, and no significant metabolites have been identified.
Excretion: Metformin is primarily eliminated by the kidneys via glomerular filtration and tubular secretion. Renal clearance is approximately 3-4 times greater than creatinine clearance, indicating active tubular secretion. In animals with renal impairment, clearance is reduced, leading to accumulation and increased risk of lactic acidosis.
Half-Life: In dogs, the elimination half-life is approximately 3-5 hours; in cats, it is approximately 4-6 hours. In humans, the half-life is about 6.2 hours.
Bioavailability: Oral bioavailability is approximately 50-60% in humans; in dogs, it may be lower (around 30-40%) due to incomplete absorption. Food reduces absorption.
Protein Binding: Metformin is negligibly bound to plasma proteins (approximately 0-5%).

Available Formulations & Strengths

Oral Tablet 500 mg, 850 mg, 1000 mg (PO)
Oral Solution 500 mg/5 mL (as hydrochloride) (PO)
Extended-Release Tablet 500 mg, 750 mg, 1000 mg (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to metformin or any component
  • Renal impairment (creatinine clearance < 60 mL/min in humans; adjust in animals)
  • Metabolic acidosis, including diabetic ketoacidosis
  • Hepatic insufficiency
  • Cardiovascular or respiratory insufficiency (risk of hypoxia)
  • Dehydration or severe infection
  • Use in animals with a history of lactic acidosis
Warnings & Clinical Precautions:
  • Use with caution in animals with renal or hepatic disease; monitor renal function before and during therapy.
  • Discontinue temporarily in animals with severe illness, dehydration, or undergoing surgery.
  • Monitor for signs of lactic acidosis (vomiting, lethargy, respiratory distress, bradycardia).
  • May cause gastrointestinal upset; administer with food.
  • Use in pregnant or lactating animals only if clearly needed; safety not established.
  • In horses, use with caution due to risk of colic and lactic acidosis.
  • Not approved for veterinary use; extra-label use requires veterinary supervision.

Adverse Effects & Reactions

Common:

  • Anorexia
  • Vomiting
  • Diarrhea
  • Lethargy
  • Nausea (in humans)

Serious / Severe:

  • Lactic acidosis (rare but potentially fatal)
  • Hypoglycemia (when used with insulin or other antidiabetics)
  • Renal impairment (worsening)

Rare:

  • Vitamin B12 deficiency (long-term use)
  • Hepatotoxicity
  • Skin reactions (urticaria, erythema)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Insulin Additive hypoglycemic effect; may require insulin dose reduction. High
Cimetidine Increases metformin plasma concentration by reducing renal clearance. Moderate
Furosemide May increase metformin plasma concentration; monitor for toxicity. Moderate
NSAIDs (e.g., meloxicam, carprofen) May reduce renal function, increasing risk of lactic acidosis. Moderate
Corticosteroids May cause hyperglycemia, reducing the efficacy of metformin. Moderate
Iodinated contrast agents Risk of acute renal failure and lactic acidosis; discontinue metformin before administration. High

Overdose & Toxicity Management

Signs of Toxicity:

  • Lactic acidosis (vomiting, abdominal pain, lethargy, respiratory distress, hypotension)
  • Hypoglycemia (rare)
  • Renal failure

Emergency Treatment Protocol: Treatment is supportive and symptomatic. Induce vomiting if recent ingestion and animal is stable. Administer activated charcoal to reduce absorption. Provide intravenous fluids to maintain hydration and renal perfusion. Correct electrolyte imbalances. In severe cases, hemodialysis may be considered to remove metformin and lactate. Monitor blood glucose, lactate, renal function, and acid-base status. Discontinue the drug and treat underlying acidosis with sodium bicarbonate if indicated.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days🥚 Eggs: 0 days

Metformin is not approved for food animals; withdrawal times are not established. Use in food animals is prohibited or requires extended withdrawal periods as per veterinary discretion and regulatory guidelines. Consult with a veterinarian for specific withdrawal recommendations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F); excursions permitted between 15-30°C (59-86°F).

Handling & Special Conditions: Keep container tightly closed. Protect from moisture. Do not freeze oral solution.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not approved for veterinary use; approved for human use.

Extra-Label (Off-Label) Use: Metformin is not FDA-approved for veterinary use. Extra-label use in animals is permitted under the Animal Medicinal Drug Use Clarification Act (AMDUCA) when a valid veterinarian-client-patient relationship exists. For food animals, extra-label use requires a withdrawal period and is prohibited if it results in violative residues. Use in horses is allowed under AMDUCA but must comply with the Food Animal Residue Avoidance Databank (FARAD) recommendations.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Metformin is used off-label in veterinary medicine primarily for the management of diabetes mellitus in cats and dogs, particularly when insulin resistance is suspected. It is not a first-line treatment; insulin remains the cornerstone of therapy. In cats, metformin may be considered as an adjunct to insulin or in early cases of type 2 diabetes, but its efficacy is variable and gastrointestinal side effects are common. In horses, it is used for equine metabolic syndrome to improve insulin sensitivity, but evidence is limited. Metformin should be used with caution in animals with renal or hepatic disease, and renal function should be monitored. It is important to educate owners on the signs of lactic acidosis and hypoglycemia. Due to the lack of veterinary approval, informed consent should be obtained, and the drug should be used only when other options are not available or have failed.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)