A drug that counters the effects of acetylcholine, a neurotransmiTTEr that stimulates muscle activity. These medications are used in Parkinson’s disease to reduce tremors and were the first drugs used to treat Parkinson’s. Anti-cholinergics commonly prescribed for Parkinson’s include trihexyphenidyl, benztropine, procyclidine, and biperiden, all of which act mainly at the mus-carinic type of acetylcholine receptor. Side effects, which increase with long-term use, can include vision problems, excessive dryness of mucous membranes, CONSTIPATION, URINARY RETENTION, and drowsiness. A recent report raises the possibility that long-term use of these anti-muscarinic agents might promote the later development of dementia.