Acetylcholine is a chemical messenger the body uses for saliva production, tear production, bladder emptying, gut movement, focusing the eye, and aspects of memory. Medicines that block its action produce a recognisable cluster of effects: dry mouth, dry eyes, blurred near vision, constipation, difficulty starting urination, flushing, and sometimes confusion.
Many first-generation antihistamines have some anticholinergic activity in addition to their antihistamine action. This is one reason clinicians take extra care when several such medicines are prescribed together, and why older adults are considered more susceptible.