Dorzolamide (Trusopt)

Dorzolamide (brand name Trusopt) is a topical carbonic anhydrase inhibitor (CAI) used to lower intraocular pressure in open-angle glaucoma and ocular hypertension. It works by suppressing aqueous humor production and is one of the standard add-on agents when a single drop is not lowering pressure enough. Dorzolamide is commonly used as part of a multi-drug regimen, especially in fixed combination with timolol as Cosopt.

How It Works

Dorzolamide blocks carbonic anhydrase II in the ciliary body. This enzyme is needed to produce aqueous humor - the fluid that maintains eye pressure. With the enzyme blocked, aqueous production drops by roughly 20%, and intraocular pressure falls about 15-20%.

The effect:

• Begins within 1-2 hours of dosing
• Reaches peak effect at 2-3 hours
• Lasts approximately 8-12 hours, requiring three-times-daily dosing as monotherapy

Like timolol, dorzolamide reduces fluid production rather than increasing drainage, which means the two drugs are additive when used together.

Common Uses

• Primary open-angle glaucoma
• Ocular hypertension
• As an adjunct when a single agent does not achieve target pressure
• Acute pressure spikes - often used with other agents

Combinations:

• Cosopt - dorzolamide + timolol; the most common dorzolamide-containing combination, FDA-labeled for patients who have not had a sufficient pressure response to a beta-blocker alone
• Cosopt PF - preservative-free Cosopt for patients with sensitivity to benzalkonium chloride

How to Use

• Monotherapy dosing: One drop in affected eye(s) three times daily
• Combination (Cosopt) dosing: One drop twice daily
• Instillation: Wash hands, tilt head back, pull down lower eyelid, apply drop
• After instillation: Close eyes for 1-2 minutes; press on the inner corner of the eye to reduce systemic absorption and minimize the metallic taste
• Multiple drops: Wait at least 5 minutes between different eye medications
• Contact lenses: Remove before use; wait 15 minutes before reinserting

Side Effects

Topical / Eye

• Stinging or burning on instillation - relatively common; usually transient
• Bitter or metallic taste - caused by drainage of the drop into the throat; reduced by punctal occlusion
• Eye redness or itching
• Allergic blepharoconjunctivitis - can develop weeks to months into therapy in some patients
• Punctate corneal staining with chronic use
• Corneal edema - uncommon overall, but more likely in patients with reduced corneal endothelial cell counts (e.g., Fuchs dystrophy or after multiple intraocular surgeries)

Systemic (uncommon, but reported)

• Headache and fatigue
• Upper respiratory or sinus symptoms
• Skin rash
• Gastrointestinal upset
• Nephrolithiasis - kidney stones; uncommon with topical use but theoretically possible due to acidification effects from systemic absorption

The systemic side effects are much less common with topical dorzolamide than with the oral CAI acetazolamide, because absorption through the eye is far less than oral dosing.

Precautions

Sulfa Allergy

Dorzolamide is a sulfonamide. The FDA label carries a class warning that sulfonamide-type adverse reactions (including rare but severe reactions like Stevens-Johnson syndrome) can occur with topical administration. However, contemporary allergy literature finds that immunologic cross-reactivity between antibiotic sulfonamides (such as sulfamethoxazole) and non-antibiotic sulfonamides like dorzolamide is uncommon, because they do not share the immunogenic structural features. Patients with a documented severe past reaction (anaphylaxis, Stevens-Johnson syndrome) to a sulfa antibiotic are usually still advised to avoid sulfonamides as a precaution; patients with a milder rash years ago can often use dorzolamide safely. Discuss your specific history with your eye doctor.

Severe Renal Impairment

• Generally avoided in patients with severe kidney disease (creatinine clearance <30 mL/min)
• Some accumulation may occur

Severe Hepatic Impairment

• Use with caution; clearance is reduced

Corneal Endothelial Disease

• Patients with Fuchs endothelial dystrophy or low endothelial cell counts (e.g., after multiple surgeries) may experience corneal edema with topical CAIs
• Use with caution and monitor corneal thickness

Pregnancy and Breastfeeding

• Discuss use with the prescribing doctor; alternative agents are usually preferred during pregnancy when clinically feasible
• Human milk transfer data are limited; the decision depends on the need for treatment and the infant's risk
• Punctal occlusion reduces systemic transfer
• Discuss alternatives with your doctor

Drug Interactions

• Oral CAIs (acetazolamide, methazolamide) - additive effects; avoid duplication
• High-dose aspirin - additive metabolic acidosis risk in vulnerable patients

Dorzolamide vs. Other Topical CAIs

The main alternative topical CAI is brinzolamide (Azopt):

Patients who cannot tolerate one CAI sometimes do well on the other. Brinzolamide tends to sting less; dorzolamide is a clear solution and is somewhat less expensive.

Dorzolamide in the Glaucoma Treatment Ladder

As monotherapy, dorzolamide produces somewhat less IOP reduction (about 15-20%) than the prostaglandin analogs (about 25-35%), so prostaglandins are usually first-line. Dorzolamide is most useful as an add-on or in fixed combination, where it adds meaningful pressure reduction on top of another agent. Most patients are not started on dorzolamide as monotherapy because:

• Three-times-daily dosing is harder to adhere to than once-daily prostaglandins
• The combination with timolol (Cosopt) covers more pressure-lowering with simpler dosing

Dorzolamide is most often added when:

• Maximum prostaglandin effect has been achieved
• A second mechanism is needed for further pressure reduction
• A patient cannot tolerate prostaglandins or beta-blockers

In patients with bilateral severe glaucoma or in those needing rapid pressure reduction, multi-class therapy commonly includes a CAI, often as Cosopt.

Frequently Asked Questions

Why does the drop give me a metallic taste?

Eye drops can drain through the tear ducts into the back of the throat and produce taste sensations. Dorzolamide and timolol-dorzolamide combinations are particularly known for a bitter or metallic taste. Pressing on the inner corner of your eye after instillation reduces drainage into the throat and minimizes the taste.

I have a sulfa allergy - can I use this?

Dorzolamide is a sulfonamide, but cross-reactivity with antibiotic sulfa drugs is thought to be uncommon. Patients with severe past reactions (anaphylaxis, Stevens-Johnson syndrome) to sulfa antibiotics are usually advised to avoid all sulfonamides as a precaution. Patients with milder histories - a rash years ago, for example - are often able to use dorzolamide safely. Tell your eye doctor about the specifics so the right decision can be made for your case.

How is this different from oral acetazolamide?

Acetazolamide is a systemic CAI given by mouth. It produces a stronger and faster pressure reduction than dorzolamide, and is used for short-term aggressive control (acute angle closure, postoperative pressure spikes) and for idiopathic intracranial hypertension. The trade-off is significant systemic side effects - paresthesias, fatigue, kidney stones, electrolyte changes. Topical dorzolamide gives most of the IOP-lowering effect with much less systemic exposure.

Can I use dorzolamide and timolol as separate drops?

You can, but most clinicians prefer the fixed combination Cosopt for adherence and convenience. Cosopt provides both medications in a single drop dosed twice daily, rather than separate drops with separated waiting times. The pharmacology is the same.

What if dorzolamide stings too much?

A few options: switch to brinzolamide (a similar topical CAI that stings less), use the preservative-free formulation (Cosopt PF), or change to a different class altogether. Some patients find that the stinging fades over the first few days; others do not, and a different agent is more comfortable long-term.