Timolol
Timolol is a nonselective beta-adrenergic blocker used topically for ocular pressure control. Veterinary applications commonly involve combination treatment with a carbonic anhydrase inhibitor. It also has a prophylactic role in selected fellow eyes of dogs with unilateral primary glaucoma. The magnitude of pressure reduction differs among species and disease states.
Group
Ophthalmic nonselective beta-blocker
Mechanism of Action
Timolol blocks beta-adrenergic receptors in the ciliary body and reduces receptor-mediated cyclic-AMP signaling involved in aqueous secretion. Decreased aqueous-humor production lowers the volume entering the anterior chamber and can reduce intraocular pressure. Its effect complements drugs that suppress aqueous production through other pathways. Mild pupil changes can also occur in veterinary species.
Dose Rates
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Glaucoma adjunct | Reference only | TOPICAL | Every 12 h | One drop of 0.5% ophthalmic solution per affected eye; not sufficient as sole treatment for a marked acute IOP crisis. |
Cats
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Glaucoma adjunct | Reference only | TOPICAL | Every 12 h | One drop of 0.5% ophthalmic solution per affected eye; not sufficient as sole treatment for a marked acute IOP crisis. |
Horses
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Glaucoma adjunct | Reference only | TOPICAL | Every 12 h | One drop of 0.5% ophthalmic solution per affected eye; not sufficient as sole treatment for a marked acute IOP crisis. |
Indications
- Adjunctive reduction of intraocular pressure in selected canine, feline or equine glaucoma, including combination treatment with dorzolamide.
- Specialist prophylactic treatment of the fellow eye in selected dogs with unilateral primary glaucoma to delay clinical onset.
Contraindications
- Use cautiously or avoid with bronchospastic disease, clinically important bradycardia, heart block or decompensated heart failure.
Interactions
- Systemic beta-blockers and other negative chronotropes may increase bradycardia.
- Combination dorzolamide adds another IOP-lowering mechanism.
Adverse effects
- Local irritation and miosis; systemic bradycardia, hypotension or bronchoconstriction may occur.
Toxicity
Remove excess exposure and provide cardiovascular/respiratory support for significant systemic beta-blockade.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Timolol 0.5% ophthalmic solution Timolol base equivalent, commonly supplied as maleate; do not substitute maleate salt mass without an equivalence statement. | Other | 5 mg / mL | TOPICAL |
Monitoring and Administration
Check IOP response, pulse and respiratory status. Systemic absorption can matter even after ocular dosing.
The source reports 0.25% as minimally effective in animals compared with 0.5%; do not infer equal efficacy from the human experience.