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Methazolamide for Dogs and Cats

By Barbara Forney, VMD

Commonly prescribed for: Medical Management of Primary and Secondary Glaucoma

Species: Dogs and Cats

Therapeutic Class: Carbonic Anhydrase Inhibitor (CAI)


Basic Information

Methazolamide is a carbonic anhydrase inhibitor (CAI) that is used in the medical management of both primary and secondary glaucoma. Carbonic anhydrase is an enzyme that is necessary for the production of aqueous humor. Methazolamide inhibits this enzyme, resulting in a decrease of the production of aqueous humor, and lower intraocular pressure. Other effects of CAI’s include increased renal excretion of sodium, potassium, and bicarbonate. Animals that are receiving CAI’s will have increased urinary pH, increased urine volume, and a tendency towards metabolic acidosis.

Dogs and Cats

Oral methazolamide is used in the acute and long-term management of both primary and secondary glaucoma in dogs. It is less commonly used in cats as they are more likely to have unacceptable side-effects with the CAI drugs. Methazolamide is absorbed from the digestive tract with significantly increased levels present at three to six hours post-administration. Research in normal dogs indicates that there is some diurnal variation in intraocular pressure (IOP). The morning IOP is generally higher than the evening pressure. Some animals can be managed with once a day medication and others will need twice a day treatment. Methazolamide is frequently used with other drugs such as prostaglandin analogues, topical miotics, and beta blockers.

Methazolamide may also be used as a part of the medical management of anterior uveitis in dogs if intraocular pressure is elevated in the affected eye.

Side Effects

  • The most-common side effects due to methazolamide use are gastrointestinal disturbance, hypokalemia, metabolic acidosis, and tachypnea.
  • Other less-common side effects include sedation or depression, bone marrow depression, kidney and liver dysfunction, polyuria, polydipsia, and rashes or hypersensitivity reactions.
  • Most side-effects can be managed through dose reduction.
  • Combining methazolamide with other topical CAI drugs may increase the likelihood of adverse effects without further improvement of the intraocular pressure.

Precautions

  • AI’s should not be used in animals with significant liver or kidney dysfunction, adrenocortical insufficiency, or significant pulmonary obstruction.
  • CAI’s should not be used in animals with electrolyte imbalances, particularly low sodium or potassium levels, or hyperchloremic acidosis.
  • Methazolamide should be avoided in pregnant animals. It is not known if methazolamide is excreted in milk, although a similar CAI, acetazolamide, is found in maternal milk.

Drug Interactions

  • Methazolamide may decrease the excretion of tricyclic antidepressants and quinidine.
  • Methazolamide should be used with caution with other drugs that may affect potassium levels. These include diuretics, corticosteroids, amphotericin B, and corticotrophin.
  • When methazolamide is used concurrently with digoxin, there may be an increased risk for toxicity due to hypokalemia.
  • Methazolamide changes urinary pH and may decrease the effects of methenamine compounds.
  • Methazolamide may decrease levels of phenobarbital and primidone.
  • Methazolamide may decrease the effects of insulin.
  • Salicylates such as aspirin may increase the risk of excess accumulation of methazolamide, potentially leading to metabolic acidosis and toxicity.

Overdose

There is no specific information regarding overdose of methazolamide. Overdose would potentially increase the likelihood and severity of the previously mentioned side-effects.

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