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Ocular Hypotensive Agents: When used in conjunction with topical miotics, latanoprost, and/or topical or systemically administered carbonic anhydrase inhibitors, the effect of timolol in lowering IOP may be additive. This effect may be used to therapeutic advantage in the treatment of glaucoma. While therapy with timolol in fixed combination with dorzolamide twice daily is associated with greater decreases in IOP than monotherapy with timolol 0.5% twice daily or dorzolamide 2% three times daily, therapy with timolol 0.5% twice daily in combination with dorzolamide 2% three times daily is associated with a slightly greater decrease in IOP (1 mmHg) than the twice-daily regimen of timolol in fixed combination with dorzolamide.
Systemic β-Adrenergic Blocking Agents: The possibility of an additive effect on IOP and/or systemic β-adrenergic blockade should be considered in patients who are receiving a systemic β-adrenergic blocking agent and topical timolol concomitantly.
Catecholamine-depleting Drugs: When topical timolol is administered concomitantly with a catecholamine-depleting drug (e.g., reserpine), the patient should be observed closely for possible additive effects and the production of hypotension and/or marked bradycardia, which may result in vertigo, syncope, and/or postural hypotension.
Other Cardiovascular Drugs: Concomitant administration of β-adrenergic blocking agent and a calcium-channel blocking agent and a cardiac glycoside may have additive effects on prolonging AV conduction. Because AV conduction disturbances, left ventricular failure, and/or hypotension may occur, caution should be exercised if timolol and a calcium-channel blocking agent are used concomitantly, and such concomitant use should be avoided in patients with impaired cardiac function. Severe bradycardia (e.g., 36 bpm), which was associated with a wandering pacemaker in one patient, and transient asystole have been reported when ophthalmic timolol and oral verapamil were used concomitantly. A single IV dose of atropine was effective in managing serious bradycardia in at least one patient. Verapamil should be used with extreme caution in patients receiving ophthalmic timolol; when therapy with a calcium-channel blocking agent is indicated (e.g., for angina) in such patients, an agent with minimal effects on SA node and cardiac conduction (e.g., nifedipine) should be used if possible.
Sinus bradycardia, which recurred upon rechallenge, has been reported when ophthalmic timolol and oral quinidine were used concomitantly. This interaction has been attributed to inhibition of timolol metabolism (via the cytochrome P-450 [CYP] 2D6 isoenzyme) by quinidine. Although oral β-adrenergic blocking agents may exacerbate rebound hypertension that may occur following discontinuance of clonidine, such an effect has not been reported in patients receiving ophthalmic timolol.
