Before using arformoterol, tell your doctor or pharmacist if you are allergic to it; or to formoterol; or if you have any other allergies. This product may contain inactive ingredients, 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: diabetes, heart problems (such as irregular heartbeat, angina, previous heart attack), high blood pressure, liver problems, seizures, overactive thyroid (hyperthyroidism), swelling of an artery (aneurysm), a certain tumor of the adrenal gland (pheochromocytoma).Arformoterol may cause a condition that affects the heart rhythm (QT prolongation). QT prolongation can rarely cause serious (rarely fatal) fast/irregular heartbeat and other symptoms (such as severe dizziness, fainting) that need medical attention right away.The risk of QT prolongation may be increased if you have certain medical conditions or are taking other drugs that may cause QT prolongation. Before using arformoterol, tell your doctor or pharmacist of all the drugs you take and if you have any of the following conditions: certain heart problems (heart failure, slow heartbeat, QT prolongation in the EKG), family history of certain heart problems (QT prolongation in the EKG, sudden cardiac death).Low levels of potassium or magnesium in the blood may also increase your risk of QT prolongation. This risk may increase if you use certain drugs (such as diuretics/"water pills") or if you have conditions such as severe sweating, diarrhea, or vomiting. Talk to your doctor about using arformoterol safely.This drug may make you dizzy. Alcohol or marijuana (cannabis) can make you more dizzy. Do not drive, use machinery, or do anything that needs alertness until you can do it safely. Limit alcoholic beverages. Talk to your doctor if you are using marijuana (cannabis).Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products).Older adults may be more sensitive to the side effects of this drug, especially QT prolongation (see above).During pregnancy, this medication should be used only when clearly needed. Discuss the risks and benefits with your doctor.It is unknown if this drug passes into breast milk. Consult your doctor before breast-feeding.