The invention describes novel methods for treating and preventing dementia caused by vascular diseases; dementia associated with Parkinson's disease; Lewy Body dementia; AIDS dementia; mild cognitive impairments; age-associated memory impairments; cognitive impairments and/or dementia associated with neurologic and/or psychiatric conditions, including epilepsy, brain tumors, brain lesions, multiple sclerosis, Down's syndrome, Rett's syndrome, progressive supranuclear palsy, frontal lobe syndrome, and schizophrenia and related psychiatric disorders; cognitive impairments caused by traumatic brain injury, post coronary artery by-pass graft surgery, electroconvulsive shock therapy, and chemotherapy, administering a therapeutically effective amount of at least one of the cholinesterase inhibitor compounds described herein. The invention also describes novel methods for treating and preventing delirium, Tourette's syndrome, myasthenia gravis, attention deficit hyperactivity disorder, autism, dyslexia, mania, depression, apathy, and myopathy associated with diabetes by administering a therapeutically effective amount of at least one of the cholinesterase inhibitor compounds described herein. The invention also describes novel methods for delaying the onset of Alzheimer's disease, for enhancing cognitive functions, for treating and preventing sleep apnea, for alleviating tobacco withdrawal syndrome, and for treating the dysfunctions of Huntington's Disease by administering a therapeutically effective amount of at least one of the cholinesterase inhibitor compounds described herein. A preferred cholinesterase inhibitor for use in the methods of the invention is donepezil hydrochloride or ARICEPT.RTM.. The invention also provides orally administrable liquid dosage formulations comprising cholinesterase inhibitor compounds, such as ARICEPT.RTM..