Q: Digoxin despite having normal (therapeutic) level can cause delirium?
Objective of above question is to highlight the complexity of delirium. Any drug, environmental discordance, pain or any underlying disease process can cause delirium.
Those medications which require therapeutic level monitoring despite being at appropriate dose can still be the cause of delirium.
Francis J. Drug-induced delirium: Diagnosis and treatment. CNS Drugs 1996; 5:103.