DEAR DOCTOR K: I've been taking over-the-counter antihistamines for years to control my allergies. Now I hear I may have to worry about dementia. How real is the concern?
DEAR READER: Antihistamine drugs have "anticholinergic" (an-tee-cole-in-ER-jik) effects. That means that they have some tendency to block the action of a natural substance called acetylcholine. This substance transmits messages in the nervous system. In the brain, it is involved in learning and memory; in the rest of the body, it stimulates muscles to contract.