Methazolamide (Monograph)

Drug class: Carbonic Anhydrase Inhibitors
ATC class: S01EC05
VA class: CV703
CAS number: 554-57-4

Medically reviewed by Drugs.com on May 14, 2024. Written by ASHP.

Introduction

Carbonic anhydrase inhibitor; nonbacteriostatic sulfonamide derivative.

Uses for Methazolamide

Glaucoma

Adjunctive treatment of open-angle glaucoma. Used principally in situations involving acute IOP elevation or chronically elevated IOP that is refractory to maximal topical ocular hypotensive therapy prior to a more definitive procedure to reduce IOP. Long-term use limited by systemic adverse effects.

Adjunctive treatment of secondary glaucoma.

Short-term use in acute angle-closure glaucoma to lower IOP before appropriate laser or incisional surgery. Should not be used for long-term treatment of angle-closure glaucoma. (See Contraindications under Cautions.)

Methazolamide Dosage and Administration

Administration

Administer orally.

Dosage

Adults

Glaucoma

Open-angle or Secondary Glaucoma, Acute Angle-closure Glaucoma

Oral

50–100 mg 2 or 3 times daily. Adjust dosage based on patient response and requirements.

Cautions for Methazolamide

Contraindications

• Marked impairment of hepatic function. Cirrhosis. (See Hepatic Impairment under Cautions.)

• Depressed serum concentrations of sodium and/or potassium.

• Adrenocortical insufficiency.

• Hyperchloremic acidosis.

• Marked impairment of renal function.

• Long-term treatment of angle-closure glaucoma; further closure of the angle may occur while worsening of glaucoma is masked by lower IOP.

• Hypersensitivity to methazolamide or any ingredients in the formulation.

Warnings/Precautions

Sensitivity Reactions

Sulfonamide Sensitivity Reactions

Serious adverse events (e.g., Stevens-Johnson syndrome, toxic epidermal necrolysis, fulminant hepatic necrosis, agranulocytosis, aplastic anemia, other blood dyscrasias) associated with sulfonamide therapy possible.

Discontinue if signs of hypersensitivity, blood dyscrasias, or other serious reactions occur.

General Precautions

Respiratory Effects

Caution in patients with pulmonary obstruction, emphysema, or advanced pulmonary disease where alveolar ventilation may be impaired. Methazolamide may precipitate or aggravate acidosis in these patients.

Laboratory Monitoring

Monitor for hematologic reactions associated with sulfonamides; obtain a CBC and platelet count before therapy and periodically during therapy. Discontinue the drug if clinically important changes occur.

Monitor serum electrolytes periodically.

Specific Populations

Pregnancy

Category C.

Lactation

Not known whether methazolamide is distributed into human milk. Discontinue nursing or drug.

Pediatric Use

Safety and efficacy not established.

Hepatic Impairment

Avoid use in patients with marked hepatic impairment, including those with cirrhosis, because of the risk of developing hepatic encephalopathy. (See Contraindications.)

Renal Impairment

Avoid use in patients with marked renal impairment. (See Contraindications.)

Common Adverse Effects

Paresthesia, hearing dysfunction or tinnitus, fatigue, malaise, anorexia, altered taste, nausea, vomiting, diarrhea, polyuria, drowsiness, confusion.

Drug Interactions

Specific Drugs and Laboratory Tests

Methazolamide Pharmacokinetics

Absorption

Bioavailability

Well absorbed from the GI tract, with peak plasma concentrations usually attained within 1–2 hours.

Onset

Reduction in IOP occurs in 2–4 hours; peak effect occurs in 6–8 hours.

Duration

Reduction in IOP persists for 10–18 hours.

Distribution

Extent

Distributed into erythrocytes, extracellular fluid, bile, the aqueous humor of the eye, and CSF.

Crosses placenta in unknown quantities.

Not known whether methazolamide is distributed into human milk.

Plasma Protein Binding

55%.

Elimination

Metabolism

Partially metabolized in liver.

Elimination Route

About 20–30% of a dose is excreted in urine as active metabolites. Fate of the remainder of the dose not determined.

Half-life

14 hours.

Stability

Storage

Oral

Tablets

Tight container at 20–25°C.

Actions

• Noncompetitive reversible inhibitor of the carbonic anhydrase enzyme.

• Reduces the formation of hydrogen and bicarbonate ions from carbon dioxide and water, thereby reducing availability of these ions for active transport into secretions.

• Decreases aqueous humor secretion and IOP.

• Increases urinary excretion of bicarbonate, sodium, and potassium due to decrease in hydrogen ions in the renal tubules. Decreases reabsorption of water, increases urine volume, urine becomes alkaline. Plasma bicarbonate concentration is decreased and chloride concentration is increased, resulting in metabolic acidosis. The manufacturer states that methazolamide should not be used as a diuretic.

• Systemically administered carbonic anhydrase inhibitors have anticonvulsant activity in animals. The manufacturer states that methazolamide is not considered an effective anticonvulsant.

Advice to Patients

• Risk of adverse effects, including sensitivity reactions; discontinue therapy and consult clinician if signs of sensitivity occur.

• Advise patients with pulmonary obstruction or emphysema that the drug may precipitate or aggravate acidosis.

• Importance of informing clinicians of existing or contemplated concomitant therapy, including prescription and OTC drugs (e.g., high-dose aspirin), as well as concomitant diseases.

• Importance of women informing clinicians if they are or plan to become pregnant or plan to breast-feed.

• Importance of informing patients of other precautionary information. (See Cautions.)

Preparations

Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.

Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.

* available from one or more manufacturer, distributor, and/or repackager by generic (nonproprietary) name

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