Dorzolamide Hydrochloride — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	Ophthalmic (topical)	One drop of 2% solution	q8h (every 8 hours)	Duration: Chronic, as needed to control IOP Notes: May be used in combination with other antiglaucoma drugs (e.g., beta-blockers, prostaglandin analogs). Monitor IOP regularly.
Cat	Ophthalmic (topical)	One drop of 2% solution	q8h (every 8 hours)	Duration: Chronic, as needed to control IOP Notes: Use with caution in cats with renal impairment. Monitor IOP.
Horse	Ophthalmic (topical)	One drop of 2% solution	q8-12h	Duration: As needed Notes: Off-label use; limited data. Use with caution.
Rabbit	Ophthalmic (topical)	One drop of 2% solution	q8-12h	Duration: As needed Notes: Off-label use; limited data.

Clinical Indications & Species Uses

General Indications

Reduction of intraocular pressure in animals with ocular hypertension or glaucoma

Dog (Canine)

Treatment of elevated intraocular pressure associated with glaucoma
Adjunctive therapy for primary or secondary glaucoma
Preoperative management of acute glaucoma

Cat (Feline)

Treatment of elevated intraocular pressure associated with glaucoma
Adjunctive therapy for primary or secondary glaucoma

Pharmacology & Mechanism of Action

Drug Class: Carbonic Anhydrase Inhibitor | Pharmacological Group: Ophthalmic Antiglaucoma Agent

Mechanism of Action: Dorzolamide is a potent, reversible inhibitor of carbonic anhydrase (CA), particularly CA-II isoenzyme, which is the predominant isoenzyme in the ciliary processes of the eye. By inhibiting CA, dorzolamide reduces the formation of bicarbonate ions, leading to a decrease in aqueous humor production. This reduction in aqueous humor formation lowers intraocular pressure (IOP) in patients with elevated IOP, such as those with glaucoma or ocular hypertension. The drug acts locally when applied topically to the eye, and its effect is additive to other antiglaucoma agents that increase aqueous outflow.

Pharmacodynamics: Dorzolamide reduces intraocular pressure by decreasing aqueous humor secretion. The onset of action occurs within 1-2 hours after topical administration, with peak effect at 2-3 hours. The duration of action is approximately 8-12 hours, allowing for twice or thrice daily dosing. In veterinary patients, the IOP-lowering effect is typically 20-30% reduction from baseline. The drug does not affect pupil size or accommodation, and it does not have significant systemic effects when used topically at recommended doses.

⚡ Pharmacokinetics Summary

Absorption: When applied topically to the eye, dorzolamide is absorbed through the cornea. Systemic absorption is minimal but can occur, especially if the drug is administered frequently or in animals with compromised corneal epithelium. In humans, systemic absorption is low enough that clinically significant systemic effects are rare. In veterinary species, similar absorption characteristics are expected.

Distribution: Dorzolamide distributes into the aqueous humor and ocular tissues. It binds to carbonic anhydrase in red blood cells, which may lead to accumulation in erythrocytes. Systemic distribution is limited due to low systemic absorption. The drug does not significantly cross the blood-brain barrier.

Metabolism: Dorzolamide is metabolized in the liver to an inactive metabolite, N-desethyl-dorzolamide. This metabolite also inhibits carbonic anhydrase but to a lesser extent. The metabolism is primarily hepatic, but the drug is also metabolized in the kidney to some degree.

Excretion: The drug and its metabolites are excreted primarily in the urine. In humans, approximately 70% of the dose is excreted in the urine as unchanged drug and metabolites. In animals, similar renal excretion is expected. Because of binding to red blood cells, the elimination half-life from blood is prolonged, but the ocular effect is determined by the concentration in the aqueous humor.

Half-Life: The ocular half-life is approximately 2-4 hours in dogs and cats. The systemic half-life is longer due to red blood cell binding, but this is not clinically relevant for topical use.

Bioavailability: Systemic bioavailability after topical ocular administration is low, typically less than 5% in humans. In veterinary species, similar low systemic bioavailability is expected.

Protein Binding: Dorzolamide is approximately 33% bound to plasma proteins. However, it binds extensively to carbonic anhydrase in red blood cells, which is a different binding site.

Available Formulations & Strengths

Ophthalmic Solution 2% (20 mg/mL) (Ophthalmic (topical))

Ophthalmic Solution (combination with timolol) 2% dorzolamide / 0.5% timolol (Ophthalmic (topical))

Contraindications & Clinical Warnings

Contraindications:

Known hypersensitivity to dorzolamide or any component of the formulation
Severe renal impairment (increased risk of metabolic acidosis)
Hyperchloremic acidosis
Concurrent use with oral carbonic anhydrase inhibitors (e.g., acetazolamide) may increase risk of systemic effects

Warnings & Clinical Precautions:

Use with caution in patients with hepatic impairment.
Use with caution in patients with a history of renal calculi (sulfonamide-type drug may cause crystalluria).
In animals with corneal endothelial compromise, use with caution as it may cause corneal edema.
Do not administer while wearing contact lenses (if applicable).
Safety in pregnant or lactating animals has not been established; use only if clearly needed.
Monitor intraocular pressure periodically to assess efficacy.
If used in combination with other topical ophthalmic drugs, separate administration by at least 5 minutes.

Adverse Effects & Reactions

Common:

Transient ocular stinging or burning
Blurred vision
Conjunctival hyperemia
Superficial punctate keratitis
Increased lacrimation

Serious / Severe:

Corneal edema
Iritis
Uveitis
Metabolic acidosis (with systemic absorption, especially in small animals or with high doses)
Contact dermatitis (rare)

Rare:

Allergic reactions
Taste perversion (in humans; not applicable to animals)
Renal calculi (with prolonged systemic exposure)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Oral carbonic anhydrase inhibitors (e.g., acetazolamide)	Additive systemic carbonic anhydrase inhibition may lead to metabolic acidosis and electrolyte imbalances.	High
Topical beta-blockers (e.g., timolol)	Additive reduction in intraocular pressure; may be beneficial but can increase systemic absorption of both drugs.	Moderate
Corticosteroids (topical or systemic)	May increase intraocular pressure, counteracting the effect of dorzolamide.	Moderate
Atropine or other anticholinergics	May increase intraocular pressure, reducing the efficacy of dorzolamide.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Ocular irritation
Systemic signs if significant absorption: metabolic acidosis, hyperchloremia, hypokalemia, lethargy, disorientation, respiratory depression

Emergency Treatment Protocol: Treatment is symptomatic and supportive. For ocular overdose, flush the eye with copious amounts of sterile saline or water. If systemic signs occur, monitor acid-base status and electrolytes, and provide supportive care such as intravenous fluids and electrolyte replacement. In severe cases, consider administration of sodium bicarbonate for acidosis. There is no specific antidote.

Food Animal Withdrawal Times

Not approved for use in food animals. Withdrawal times are not established. If used off-label in food animals, consult a veterinarian and follow appropriate withdrawal periods based on pharmacokinetic data and regulatory guidelines.

Storage, Handling & Regulatory Information

Storage Temperature: Store at 15-30°C (59-86°F). Protect from freezing.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Keep container tightly closed. Protect from light. Do not use if solution changes color or becomes cloudy.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in humans; not specifically approved for veterinary use, but commonly used off-label in veterinary medicine.

Extra-Label (Off-Label) Use: In the US, extra-label use of dorzolamide in food animals is prohibited under AMDUCA because it is not approved for use in food animals and no tolerance has been established. In non-food animals, extra-label use is permitted under veterinary supervision.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Dorzolamide is a valuable adjunctive therapy for managing glaucoma in dogs and cats. It is often used when beta-blockers alone are insufficient or contraindicated. The drug is generally well-tolerated, but local irritation is common. In veterinary patients, it is important to monitor intraocular pressure regularly and adjust dosing frequency based on response. Combination products with timolol may improve compliance. Systemic side effects are rare but can occur, especially in small patients or with frequent dosing. Use with caution in animals with renal or hepatic disease. For food animals, this drug is not recommended due to lack of withdrawal data and regulatory restrictions.

References & Literature

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