Acetylcholine is one of the body's many neurotransmitters. A neurotransmitter is a chemical substance that nerve cells use to communicate with each other, with muscles, and with many glands. Drugs that block the action of the neurotransmitter acetylcholine are said to have anticholinergic effects. Most of these drugs aren't designed to block acetylcholine; their anticholinergic effects are side effects.
Elderly people are particularly sensitive to drugs with anticholinergic effects because the amount of acetylcholine in the body decreases with age and because their bodies are less able to use what's there. Drugs that have anticholinergic effects can cause confusion, blurred vision, constipation, dry mouth, light-headedness, and difficulty with urination or loss of bladder control.
