Methazolamide
N-(4-methyl-2-sulfamoyl-Δ2-1,3,4-thiadiazolin-5-ylidene)acetamide
Also known as: Neptazane · GlaucTabs · Glaumetax · MZM · Methazolamidum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Medical treatment of glaucoma | 2.5-5 mg/kg PO q8-12h | PO | q8-12h | — | 🌎 NA |
| Medical treatment of glaucoma | 2-4 mg/kg PO two to three times a day | PO | two to three times a day | — | 🌎 NA |
| Medical treatment of glaucoma | 3-5 mg/kg divided q12h PO | PO | q12h | — | 🌎 NA |
| Medical treatment of glaucoma | 2-5 mg/kg PO two to three times a day | PO | two to three times a day | — | 🌎 NA |
| Medical treatment of glaucoma | 2.2-4.4 mg/kg PO two to three times a day. | PO | two to three times a day | — | 🌎 NA |
| Medical treatment of glaucoma | 2-10 mg/kg PO two to three times a day. | PO | two to three times a day | — | 🌎 NA |
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Medical treatment of glaucoma | 3-4 mg/kg PO twice a day. | PO | twice a day | — | 🌎 NA |
| Medical treatment of glaucoma | 1-2 mg/kg PO q8-12h | PO | q8-12h | — | 🌎 NA |
- Medical treatment of glaucoma: Cats may not tolerate oral carbonic anhydrase inhibitors (CAIs) as well as dogs. Reported side effects include lethargy, inappetence, vomiting. Topical CAIs may be better tolerated.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Methazolamide is an oral carbonic anhydrase inhibitor (CAI) primarily utilized in veterinary medicine for the medical management of open-angle glaucoma.
Historically a mainstay for reducing intraocular pressure (IOP), systemic CAIs like methazolamide have largely been supplanted by topical formulations (e.g., dorzolamide, brinzolamide). Topical agents offer comparable IOP reduction with a significantly lower incidence of systemic adverse effects.
Key clinical points:
- Systemic Effects: Because it is administered orally, methazolamide affects carbonic anhydrase throughout the body, leading to potential metabolic acidosis and electrolyte imbalances (especially hypokalemia).
- Clinical Utility: It is typically reserved for cases of glaucoma that are refractory to topical therapies or when topical administration is not feasible.
- Species Sensitivity: Cats are particularly sensitive to the adverse effects of oral CAIs and may exhibit profound lethargy and anorexia.
Mechanism of action
Methazolamide acts via noncompetitive, reversible inhibition of the enzyme carbonic anhydrase (CA), specifically the CA-II isoenzyme.
- In the Eye (Ciliary Body): Inhibition of CA → reduces the formation of hydrogen (H+) and bicarbonate (HCO3-) ions from carbon dioxide and water → decreases the availability of these ions for active transport into the posterior chamber → decreased aqueous humor production → reduced intraocular pressure (IOP).
- In the Kidneys (Proximal Tubule): Inhibition of CA → increased renal tubular secretion of sodium (Na+), potassium (K+), and bicarbonate (HCO3-) → increased urine alkalinity and volume, often resulting in a mild hyperchloremic metabolic acidosis.
- In the CNS: Exhibits anticonvulsant activity independent of its diuretic effects, potentially due to localized CA inhibition in the brain or secondary to systemic metabolic acidosis.
Safety & warnings
Contraindications
- Significant hepatic disease (may precipitate hepatic coma)
- Renal or adrenocortical insufficiency
- Hyponatremia or hypokalemia
- Hyperchloremic acidosis or severe electrolyte imbalance
- Severe pulmonary obstruction (inability to increase alveolar ventilation to compensate for metabolic acidosis)
- Hypersensitivity to carbonic anhydrase inhibitors or sulfonamides
- Chronic, noncongestive, angle-closure glaucoma (long-term use may mask the condition by lowering IOP while angle closure progresses)
Adverse effects
- GI disturbances (vomiting, diarrhea, inappetence)
- Metabolic acidosis (often manifesting as heavy panting in dogs)
- CNS effects (sedation, depression, disorientation, excitement)
- Hematologic effects (bone marrow depression, thrombocytopenia)
- Renal effects (crystalluria, dysuria, renal colic, polyuria, polydipsia)
- Hypokalemia
- Hyperglycemia
- Hyponatremia
- Hyperuricemia
- Hepatic insufficiency
- Dermatologic effects (rash)
- Hypersensitivity reactions
- Weakness or cardiac arrhythmias (secondary to electrolyte imbalances)
Precautions
Laboratory Interferences: By alkalinizing the urine, carbonic anhydrase inhibitors may cause false-positive results in determining urine protein using bromphenol blue reagent, sulfosalicylic acid, nitric acid ring test, or heat and acetic acid test methods. They may also decrease iodine uptake by the thyroid gland in hyperthyroid or euthyroid patients.
- Use with caution in pregnant animals (FDA Category C in humans; animal studies show adverse fetal effects).
- Combining oral methazolamide with topical dorzolamide does not yield additive reductions in IOP and may increase adverse effects.
- Monitor closely for signs of hypokalemia and metabolic acidosis, especially in cats.
Drug interactions
Alkaline urine caused by methazolamide may decrease excretion of tricyclic antidepressants.
Increased risk of methazolamide accumulation and toxicity; increased risk for metabolic acidosis; methazolamide increases salicylate excretion.
Methazolamide may cause hypokalemia, leading to an increased risk for digoxin toxicity.
Rarely, carbonic anhydrase inhibitors may interfere with the hypoglycemic effects of insulin.
Methazolamide alkalinizes urine, which may negate the urinary antiseptic effects of methenamine.
Concomitant use may exacerbate potassium depletion and hypokalemia.
Increased urinary excretion due to alkaline urine, which may reduce phenobarbital serum levels.
May result in decreased primidone concentrations.
Alkaline urine caused by methazolamide may decrease quinidine excretion.
Monitoring
- Intraocular pressure (IOP) / tonometry
- Serum electrolytes (especially potassium and sodium)
- Blood pH / acid-base status
- Baseline CBC with differential, and periodic retests if using chronically
- Clinical signs of adverse effects (lethargy, GI upset, heavy panting)
Pharmacokinetics
Absorption
Absorbed from the GI tract, albeit more slowly than acetazolamide.
Distribution
Distributed throughout the body, including the cerebrospinal fluid (CSF) and aqueous humor.
Metabolism
At least partially metabolized in the liver.
Elimination
Excreted primarily via the kidneys, leading to increased urine alkalinity.
Overdose
Information regarding acute overdosage of methazolamide in veterinary species is limited.
Clinical Signs: Expected signs of toxicity include severe electrolyte imbalances (hypokalemia, hyponatremia), profound metabolic acidosis, CNS depression or disorientation, and dehydration secondary to diuresis.
Management:
- Monitor serum electrolytes, blood gases (for acidosis), volume status, and CNS status.
- Treat symptomatically and supportively.
- Administer intravenous fluids to correct dehydration and electrolyte deficits (especially potassium supplementation if indicated).
- Correct severe metabolic acidosis with sodium bicarbonate if necessary, based on blood gas analysis.
Available products
Formulations
- Tablets
Human-labeled
- Methazolamide Tablets: 25 mg & 50 mg (generic, Rx)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Methazolamide tablets should be stored at room temperature in well-closed containers. Tablets typically have an expiration date of 5 years after manufacture.
Client information
Understanding the Medication: Methazolamide is used to decrease the pressure inside your pet's eye(s) caused by glaucoma. It works by reducing the amount of fluid the eye produces.
Administration:
- Give with food: If this medication causes stomach upset (vomiting or loss of appetite), giving it with a meal can help.
- Do not crush or chew unless directed by your veterinarian.
What to Watch For:
- Heavy Panting: This drug can cause a mild change in blood acidity (metabolic acidosis), which dogs often try to correct by panting heavily. Contact your vet if this seems excessive.
- Lethargy and Hiding: Cats are especially sensitive to this medication and may become very tired or stop eating.
- Increased Thirst and Urination: You may notice your pet drinking and urinating more than usual.
- Seek immediate veterinary care if you notice abnormal bleeding or bruising, severe weakness, muscle tremors, or a skin rash.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.