However, research findings to date are still controversial. Some studies have found a significant association between use of ACH medications and delirium,23,27- 30 whereas others have not.12- 14 Several reasons may underlie this discrepancy. First, studies used different measures of ACH medication exposure, including serum ACH level,26- 29 aggregate risk scores of ACH potency,27,28,30- 33 or number and dose of ACH medications using different classifications.12- 14,17,18 Second, the effect of ACH medications on delirium may be confounded by other risk factors, such as dementia, age, or comorbid conditions. Third, patients with dementia showed cognitive decline with doses of ACH medications at which nondemented controls did not,16 suggesting that dementia modifies the ACH-delirium relation. Finally, most published studies considered medication use to be a precipitating factor only. Exposure was typically measured before onset of delirium, either at a single point or accumulated over time to the onset of delirium.11- 14,17,18 Whether and to what degree ACH medications play a role in predicting the severity of delirium symptoms after its onset has not yet been investigated, to our knowledge. Because the types, doses, and timing of prescribed medications can change frequently, especially in hospitalized patients, and presentation and severity of delirium symptoms typically fluctuate over time, studies that ignore the dynamic features of medication exposure and delirium symptoms might be biased by a false temporal sequence or confounding by indication. Therefore, we conducted this study to investigate the effect of ACH medication exposure on the subsequent severity of delirium symptoms in a cohort of hospitalized elderly patients with diagnosed delirium. Our 2 a priori hypotheses were that (1) current exposure to ACH medications is independently associated with increased severity of delirium symptoms and (2) the effect of ACH medication exposure on delirium severity may depend on dementia status, with demented patients being more sensitive to ACH medications than those without dementia.