Indication indicated for the treatment of parkinson s disease and extrapyramidal reactions caused by drugs.mechanism of action trihexyphenidyl is a selective m1 muscarinic acetylcholine receptor antagonist. it is able to discriminate between the m1 (cortical or neuronal) and the peripheral muscarinic subtypes (cardiac and glandular). trihexyphenidyl partially blocks cholinergic activity in the cns, which is responsible for the symptoms of parkinson s disease. it is also thought to increase the availability of dopamine, a brain chemical that is critical in the initiation and smooth control of voluntary muscle movement.