Study leader Denise Boudreau, an associate scientific investigator at Group Health Research Institute in Seattle, said cholinesterase inhibitors -- such as donepezil or Aricept, which work by inhibiting the breakdown of acetylcholine -- constitute the primary therapy for slowing Alzheimer's disease.
"Anticholinergic properties are often found in drugs commonly used to treat gastrointestinal disorders, allergies, urinary incontinence, depression and Parkinson's disease, and they can have negative effects on cognition and function in the elderly," Boudreau said in a statement. "If someone is taking both types of drugs -- cholinesterase inhibitors and anticholinergic medications -- they will antagonize each other, and neither will work."
The study, published online in the Journal of the American Geriatrics Society, found 37 percent of Alzheimer's patients who were using cholinesterase inhibitors were also taking at least one anticholinergic drug -- and more than 11 percent took two or more.
Study subjects using both medication types were not more likely to enter a nursing home or to die than those taking only cholinesterase inhibitors, the study said.
"It's reassuring that we did not observe an association between simultaneous use of the two types of drugs and increased risk of death or nursing home placement," Boudreau said in a statement. "But concomitant use of these drugs is, at the very least, not optimal clinical practice."