Pharmacology · Carbonic anhydrase inhibitors

Acetazolamide !

A carbonic anhydrase inhibitor that lowers eye pressure and causes a mild acidosis. Used in glaucoma and to prevent altitude sickness.

Updated 2026-11-01

Class: Carbonic anhydrase inhibitors
Enzyme: Carbonic anhydrase II
ATC code: S01EC01
WHO Model List of Essential Medicines

Carbonic anhydrase IIAcetazolamide

The figure shows acetazolamide bound to the zinc in the active site of human carbonic anhydrase II.

Target in 3D: Carbonic anhydrase II. PDB structure 3HS4 (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Acetazolamide's sulfonamide group binds the zinc ion in the active site of carbonic anhydrase. In the proximal tubule bicarbonate can no longer be reclaimed, so bicarbonate, sodium, potassium and water leave in the urine, the urine turns alkaline and the blood becomes mildly acidic. In the eye it reduces the formation of aqueous humour.

Acetazolamide binds the zinc of carbonic anhydrase
The proximal tubule cannot reclaim bicarbonate
Alkaline urine and mild metabolic acidosis
In the eye: less aqueous humour, lower pressure

Pharmacokinetics

Absorption: Absorbed by mouth; also given IV
Distribution: Concentrates in red blood cells, which are rich in carbonic anhydrase, and binds plasma proteins
Elimination: Excreted by the kidneys; it accumulates in kidney failure

Indications

Open-angle and secondary glaucoma, as an adjunct Open-angle glaucoma →
Acute angle-closure glaucoma before surgery Acute angle-closure glaucoma →
Prevention of acute mountain sickness during rapid ascent
Some epilepsies, as an adjunct Epilepsy and seizures →
Oedema from heart failure or drugs, as an adjunct Heart failure →

Adverse effects

Common

Tingling in the hands, feet and face
Altered taste
Drowsiness and fatigue
Loss of appetite, nausea
Increased urination

Contraindications

Hypersensitivity to acetazolamide or sulfonamides
Low sodium or potassium
Marked kidney or liver disease, including cirrhosis (risk of hepatic encephalopathy)
Adrenal insufficiency
Hyperchloraemic acidosis
Long-term use in chronic angle-closure glaucoma

Key interactions

High-dose aspirin: Severe acidosis, lethargy, coma and death have been reported
Phenytoin: Higher phenytoin levels and more osteomalacia
Lithium: More lithium excretion and lower lithium levels
Amphetamines: Alkaline urine reduces their excretion and prolongs their effect

What is monitored

Electrolytes, especially potassium
Blood pH or bicarbonate
Complete blood count
Kidney function

Nursing care

Ask about sulfonamide allergy before the first dose.
Check potassium and bicarbonate as ordered; report weakness or rapid breathing.
Teach that tingling and altered taste are common, and to report rash, sore throat, fever or unusual bleeding.
Teach to drink enough fluids to lower the risk of kidney stones.
In altitude sickness, it does not replace descent if severe symptoms appear.

Pregnancy and breastfeeding

Avoided where possible, especially early in pregnancy: it caused limb defects in animal studies.

Test yourself

Think of the answer, then tap the question to see it.

What acid-base change does acetazolamide cause?

A hyperchloraemic metabolic acidosis with alkaline urine, from loss of bicarbonate in the proximal tubule.

Why is acetazolamide contraindicated in cirrhosis?

Alkaline urine reduces ammonia excretion; the extra ammonia can cause hepatic encephalopathy.

Which interaction with aspirin is dangerous?

With high-dose aspirin, severe acidosis, lethargy, coma and death have been reported.

Why does acetazolamide help prevent acute mountain sickness?

Its metabolic acidosis stimulates breathing, which offsets the respiratory alkalosis of altitude and improves oxygenation.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - ACETAZOLAMIDE tablet (DailyMed (U.S. National Library of Medicine), 2026); Acetazolamide: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2017); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); RCSB PDB - 3HS4: Human carbonic anhydrase II complexed with acetazolamide (RCSB Protein Data Bank, 2026).

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Representative model, for educational purposes only.