Before using tetrahydrozoline, tell your doctor or pharmacist if you are allergic to it; or if you have any other allergies. This product may contain inactive ingredients (such as preservatives like benzalkonium chloride), which can cause allergic reactions or other problems. Talk to your pharmacist for more details. Before using this medication, tell your doctor or pharmacist your medical history, especially of: glaucoma, heart problems (e.g., heart attack, chest pain), high blood pressure, diabetes, eye infection/injury, overactive thyroid (hyperthyroidism). This drug may cause temporary blurred vision after you apply it. Do not drive, use machinery, or do any activity that requires clear vision until you are sure you can perform such activities safely.