The drugs, which include many household names such as codeine and warfarin, are 'anticholinergic' - an activity that prevents the brain from sending messages between nerve cells.
The most dangerous are antidepressants, tranquilizers, bladder medication, and antihistamines, researchers from the University of East Anglia discovered.
They assessed more than 80 common drugs for their anticholinergic effects, and 20 per cent of people taking a high-scoring drug, or combination of drugs, were dead within two years. This compared with just seven per cent of people who were taking drugs that were not anticholinergic.
A patient's risk of dying increased by 26 per cent for each additional point - from zero to three - that a drug was ranked for its anticholinergic effect.
People taking a combination of drugs that had a combined anticholinergic score of five or more also had diminished cognitive abilities, the researchers found.
(Source: Journal of the American Geriatrics Society, published online, June 24, 2011).