First-generation antihistamines: what side effects should you know?

These medicines block histamine, which helps allergy symptoms such as a runny or blocked nose, itchy eyes, and itchy throat. They can also affect brain pathways and other body signals, producing drowsiness and a dry mouth. Older adults may be more sensitive to confusion, falls, constipation, or urinary problems. Alcohol, sleeping tablets, sedatives, and some other medicines can increase reduced alertness.

Read the label and take only the recommended dose. Until you know how the medicine affects you, do not drive, cycle, swim alone, or operate machinery. Avoid alcohol and check with a pharmacist before combining it with a sleep or anxiety medicine. Use extra caution if you have glaucoma, prostate or bladder problems, breathing disease, or take several medicines. For ongoing allergy symptoms, ask whether another option would cause less drowsiness.

Seek urgent help for trouble breathing, swelling of the lips or tongue, collapse, severe confusion, or an inability to wake normally. Contact a clinician for a fast or irregular heartbeat, severe blurred vision, painful difficulty passing urine, or side effects that persist after the dose wears off. Never give a sedating antihistamine to a child unless the product and dose have been checked by a professional.

Ask whether your symptoms fit allergic rhinitis, allergic conjunctivitis, or another cause, and whether your current medicines make sedation more likely. Ask how long to use the product and what to do if you miss a dose or feel unusually drowsy.