The precise mechanism of action of trihexyphenidyl remains inadequately comprehended; it appears to act on the parasympathetic nervous system by inhibiting efferent impulses directly. Structures innervated by the parasympathetic system, such as the salivary glands, eyes, and smooth muscles (directly and indirectly), are affected, even at smaller doses. The direct central inhibition of cerebral motor centers may occur with higher doses. Research indicates that the receptors affected are the dopamine and M1 muscarinic receptors.