Common drugs do not lead to additional cognitive harm to Alzheimer's sufferers.
That is the main finding of research by scientists at the University of East Anglia (UEA) and other universities, who had earlier reported that some antidepressants and antihistamines had a detrimental impact on the cognitive ability of the elderly.
Common over-the-counter and prescription medicines had been shown to inhibit the effect of the neurotransmitter acetylcholine, leading to an anticholinergic effect that occasionally led to cognitive impairment and even death in older people.
However, the new study demonstrated that dementia patents taking the drugs suffer no further effects.
Dr Chris Fox, from UEA, said that despite the concerns that remain to the elderly he hoped to reassure families and carers of those with dementia.
"A low dose of one medication with a low degree of anticholinergic activity does not appear to lead to more impaired cognition or a more rapid cognitive decline in people with dementia over the next six or 18 months," he added.
Doctors believe that because the cholinergic system is already impaired by dementia the anti-cholinergic effect of the medication is negligible.
Figures from The Alzheimer's Society show there are currently 750,000 people living with dementia in the UK.