Hundreds of thousands of older people who take common drugs could be at increased risk of mental decline and death, research suggests.
A University of East Anglia-led study of 13,000 people aged 65 and over found risks from over-the-counter and prescription drugs, including some antihistamines, painkillers, blood thinners and eye drops for glaucoma.
The research, published in the Journal of the American Geriatrics Society, suggests that well-known side-effects of the drugs could have an impact on the brain, increasing the risk of decline and death.
The experts analysed more than 80 drugs for "anticholinergic activity", a potential side effect which affects the brain by blocking a key neurotransmitter called acetylcholine. The drugs were ranked from one to three according to the strength of this activity, with drugs scoring one having a mild effect, two a moderate effect and three causing the most serious concern.
Taking a combination of drugs - such as four scored at one each - increased the risks, with experts saying the threshold for damage in patients was a total score of about four. The risk was cumulative, based on the number of drugs taken and the strength of each drug's effect.
Some of the most dangerous (score three) drugs are commonly available, including the antihistamines chlorphenamine (used in the brand Piriton) and promethazine (used in Phenergan), anti-depressants amitriptyline (used in several brands) and paroxetine (used in Seroxat) and the incontinence drug oxybutynin (used in Ditropan).
The blood thinner warfarin scored one, as did heartburn drug ranitidine (used in the brand Zantac), painkiller codeine and timolol maleate eye drops.
The results showed that one in five people taking drugs with a total score of four or more had died by the end of the two-year study, compared with only 7% of those taking no anticholinergic drugs. That represents around treble the risk. For every additional point scored, the odds of dying increased by 26%, the study also found.
Those people taking drugs with a combined score of five or more scored more than 4% lower in a cognitive function test than those taking none of the drugs.
The study forms part of the Medical Research Council's Cognitive Function and Ageing Studies project, which hopes to uncover risk factors for dementia. It included experts from the University of Cambridge and Indiana University in the US.