Donepezil-d4 (hydrochloride) (Item No. 18251) is intended for use as an internal standard for the quantification of donepezil (Item No. 13245) by GC- or LC-MS. Donepezil is a reversible acetylcholinesterase inhibitor (IC50 = 23 nM) that readily crosses the blood-brain barrier to reduce the breakdown of acetylcholine. As acetylcholine modulates plasticity, excitability, and arousal in the central nervous system, donepezil has been used to study cognition, memory, and behavior in animal models.