Donepezil, an acetylcholinesterase inhibitor used to treat Alzheimer’s disease, was developed in the late 1980s and approved for medical use in the mid-1990s. Its history is marked by its effectiveness in improving cognitive symptoms in patients with Alzheimer’s disease, though it is not a cure and primarily offers symptomatic relief by increasing acetylcholine levels in the brain.
Donepezil, a centrally acting cholinesterase inhibitor used in the management of mild to severe Alzheimer’s disease, was approved in the United States in 1996 and is widely included in dementia treatment regimens, often as a first-line symptomatic therapy. Its clinical development was supported by extensive trials assessing cognitive and functional outcomes in patients with neurodegenerative disease, along with long-term safety monitoring programs that evaluated common adverse effects such as nausea, diarrhea, insomnia, and bradycardia, which influenced prescribing guidance and patient selection.
BRAND NAMES
Aricept (the original and most widely recognized brand, marketed by Eisai and Pfizer in many countries)
Adlarity (a transdermal patch formulation of donepezil).
MECHANISM OF ACTION
Donepezil is a reversible, centrally acting acetylcholinesterase inhibitor that works by blocking the enzyme acetylcholinesterase responsible for the breakdown of acetylcholine in the synaptic cleft. In Alzheimer’s disease, where cholinergic neuron loss leads to reduced acetylcholine levels, donepezil increases the availability of acetylcholine in the brain by inhibiting its degradation, thereby enhancing cholinergic neurotransmission at neuronal synapses.
PHARMACOKINETICS
Absorption
Donepezil is well absorbed after oral administration, with high bioavailability (approximately 100%) and peak plasma concentrations typically reached within 3 to 4 hours. Food does not significantly affect the rate or extent of absorption.
Distribution
Donepezil has a large volume of distribution, approximately 12–16 L/kg, indicating extensive distribution into body tissues. It is about 96% bound to plasma proteins, primarily albumin and alpha-1-acid glycoprotein.
Metabolism
Donepezil is extensively metabolized in the liver, primarily via the cytochrome P450 enzyme system, especially CYP2D6 and CYP3A4. It undergoes both primary and secondary biotransformation to several metabolites, some of which are active but present in much lower concentrations than the parent drug.
Elimination
Donepezil is eliminated through both renal and hepatic routes after extensive metabolism. Approximately ~57% is excreted in urine (mainly as metabolites) and about ~15% is excreted in feces, with only a small fraction of the drug eliminated unchanged.
PHARMACODYNAMICS
Donepezil produces its pharmacological effects by inhibiting central acetylcholinesterase, leading to increased concentrations of acetylcholine at cholinergic synapses in the brain. This enhances cholinergic neurotransmission, particularly in regions associated with memory, learning, and attention such as the hippocampus and cerebral cortex, which are significantly affected in Alzheimer’s disease.
ADMINISTRATION
Donepezil is administered orally, available as immediate-release tablets and orally disintegrating tablets (ODTs), and in some regions as a once-daily transdermal patch formulation (Adlarity). The usual dosing is once daily, preferably taken at bedtime to reduce gastrointestinal side effects such as nausea. It can be taken with or without food, as food does not significantly affect its absorption.
DOSAGE AND STRENGTH
Donepezil is available in multiple dosage strengths depending on formulation. The most commonly used oral tablet and orally disintegrating tablet strengths are 5 mg and 10 mg for mild to moderate Alzheimer’s disease, while a higher 23 mg extended-release tablet is used in selected patients with moderate to severe disease who have tolerated lower doses.
DRUG INTERACTIONS
Donepezil is metabolized mainly by CYP2D6 and CYP3A4, so drugs that inhibit or induce these enzymes can significantly alter its plasma levels. Strong CYP3A4 inhibitors such as ketoconazole and CYP2D6 inhibitors like quinidine may increase donepezil concentrations and the risk of adverse effects, while enzyme inducers such as rifampicin, carbamazepine, or phenytoin may reduce its therapeutic effectiveness.
FOOD INTERACTIONS
Donepezil has no clinically significant food interactions, and its absorption is not meaningfully affected by food intake. It can therefore be taken with or without meals according to patient preference. However, taking donepezil with food or at bedtime is sometimes recommended in clinical practice to help reduce gastrointestinal side effects such as nausea, vomiting, and dyspepsia.
CONTRAINDICATIONS
Donepezil is contraindicated in patients with a known hypersensitivity to donepezil or to piperidine derivatives. It should also be avoided in individuals who have previously experienced severe allergic reactions such as angioedema or anaphylaxis related to the drug.
SIDE EFFECTS
Nausea
Vomiting
Diarrhea
Abdominal pain / dyspepsia
Loss of appetite (anorexia)
Weight loss
Headache
Dizziness
Insomnia
OVER DOSAGE
Donepezil overdose results from excessive cholinergic stimulation due to marked acetylcholinesterase inhibition. Clinical features are primarily manifestations of severe cholinergic toxicity, which may include pronounced nausea, vomiting, diarrhea, abdominal cramps, increased salivation, sweating, and urinary incontinence.
TOXICITY
Donepezil toxicity results from excessive acetylcholinesterase inhibition leading to marked cholinergic overstimulation in the central and peripheral nervous systems. Clinically, it presents as a cholinergic crisis characterized by severe gastrointestinal symptoms such as nausea, vomiting, diarrhea, and abdominal cramps, along with increased secretions including salivation, sweating, and lacrimation. Cardiovascular manifestations may include bradycardia, hypotension, syncope, and in severe cases heart block or circulatory collapse.
