This patient should stop taking rivastigmine. Given the results of her cognitive testing, she meets criteria for Alzheimer disease of mild severity. She began taking oral rivastigmine, a cholinesterase inhibitor, 12 weeks ago. All of the available cholinesterase inhibitors are approved for mild to moderate Alzheimer disease, except donepezil, which is also approved for the severe stage. Studies of cholinesterase inhibitors and memantine show consistent improvement on measures of cognition and global assessment of dementia, but the effect size is modest and evidence that they improve long-term outcome is lacking. In practice, individual response is variable. There is insufficient evidence to support one cholinesterase inhibitor over another, and choice of treatment in a patient should be based on cost, tolerability, and ease of using the specific formulation. There is also insufficient evidence of the optimal duration of treatment or when therapy should be discontinued. Medication decisions should be made on an individual basis. Cholinesterase inhibitors should be used with caution in patients with cardiac conduction abnormalities, active peptic ulcer disease (because of the risk of bleeding), and seizures. Gastrointestinal adverse effects are common to all cholinesterase inhibitors and include loss of appetite, weight loss, nausea, vomiting, and diarrhea. Insomnia also can occur. This patient has had a significant amount of weight loss, loss of appetite, and insomnia since starting rivastigmine. The most appropriate next step in management would be to discontinue the medication. A trial of a different type of cholinesterase inhibitor could be considered, but only after symptoms subside.

Donepezil, another cholinesterase inhibitor, might be considered as an alternative therapy for this patient. However, no indication supports prescribing multiple cholinesterase inhibitors concomitantly, and rivastigmine should be discontinued as the first step.

Memantine is a noncompetitive N-Methyl-D-aspartate receptor antagonist approved by the FDA for the treatment of moderate to severe Alzheimer disease. Although this drug could be added in the future, this patient's present symptoms should be addressed first.

Mirtazapine is a nonselective α₂-adrenoceptor antagonist effective in the treatment of depression. Stimulation of appetite, weight gain, and somnolence are frequently associated effects, and thus this medication may be the preferred treatment for depressed patients with Alzheimer disease who have insomnia or loss of appetite. This patient has apathy and loss of interest, which are common symptoms in Alzheimer disease, but lacks additional symptoms to suggest depression.