Dementia trials could be failing because they all-too-often overlook the physical health of patients - according to new research from the University of East Anglia and Aston University.
More than 60 per cent of people with dementia are estimated to have three or more other conditions (co-morbidities).
The research shows how the combined effects of co-morbidities including diabetes, lung disease, arthritis and chronic heart failure are not being adequately described in dementia trials.
It investigates the extent of co-morbidities in people with dementia and the recording of medication usage in clinical trials, for the first time.
Previous research has found that medication cocktails make it difficult to show whether new dementia drugs are working.
Researchers hope that the findings will lead to better reporting of co-morbidities and medication use in future trials, and benefit the search for a cure.
There are 36 million people worldwide with Alzheimer's disease and other forms of dementia and this expected to double by 2030 and reach 115 million by 2050 unless a major breakthrough is made.
Lead researcher Dr Chris Fox said: "To date, research on the specific physical healthcare needs of people with dementia has been neglected.
"We took into account nine trials, all of which had recorded data about comorbidities. But a further 15 studies had not recorded data on comorbidities. This shows that dementia trials are failing - which is why we are not making progress.
"Falls, malnutrition, frailty, incontinence, sleep disorders and sight problems are found to occur more frequently in dementia sufferers and untreated can lead to more severe health problems, pain and distress, as well as worsening the symptoms of dementia. And as the severity of the dementia worsens, so does the rate of comorbid conditions.
"But many physical comorbidities are often treatable and some may be reversible. Pneumonia, urinary tract infection, congestive cardiac failure and dehydration account for more than two thirds of preventable dementia admissions.
"The biggest problem is that it is often difficult for people with dementia to communicate that they have another medical complaint. This leads to poor reporting of medical comorbidities," he added.
Dr Ian Maidment from Aston University said: "We found that medication usage was poorly reported. This is important, because our previous research has found that many medications work against new drugs designed to treat dementia potentially making it difficult to prove whether or not these new drugs actually work. Ultimately this may undermine the commitment by the G8 to find new drugs to treat dementia."
The team from UEA, Aston University, the University of Hull and Bradford Institute for Health Research looked at information about 1474 people with dementia from nine randomised control trials.
The study found that comorbidity was rife - the most prevalent conditions being neurological disorders (91 per cent) vascular disorders (91 per cent), cardiac disorders (74 per cent) and depression (59 per cent).
'Systematic review investigating the reporting of comorbidities and medication in randomized controlled trials of people with dementia' and published in the September edition of Age and Ageing alongside a commentary piece 'The importance of detecting and managing comorbidities in people with dementia'.