More than 99% of drug trials for Alzheimer's disease during the past decade have failed, according to a study.
There is an urgent need to increase the number of potential therapies being investigated, say US scientists.
Only one new medicine has been approved since 2004, they report in the journal Alzheimer's Research & Therapy.
The drug failure rate is troubling and higher than for other diseases such as cancer, says Alzheimer's Research UK.
Dr Jeffrey Cummings, of the Cleveland Clinic Lou Ruvo Center for Brain Health, in Las Vegas, and colleagues, examined a public website that records clinical trials.
Between 2002 and 2012, they found 99.6% of trials of drugs aimed at preventing, curing or improving the symptoms of Alzheimer's had failed or been discontinued.
This compares with a failure rate of 81% for cancer drugs.
The failure rate was "especially troubling" given the rising numbers of people with dementia, said Dr Simon Ridley, of Alzheimer's Research UK.
"The authors of the study highlight a worrying decline in the number of clinical trials for Alzheimer's treatments in more recent years," he said.
"There is a danger that the high failure rates of trials in the past will discourage pharmaceutical companies from investing in dementia research.
"The only way we will successfully defeat dementia is to continue with high quality, innovative research, improve links with industry and increase investment in clinical trials."
Dr Eric Hill, of the School of Life and Health Sciences at Aston University, said more research was needed to understand the complex mechanisms behind the disease.
"The development of better experimental models that could be incorporated into a battery of tests, will not only help us to understand the changes that occur in the brains of Alzheimer's disease patients, but also provide tools for the development of new drug treatments that could slow or stop the onset of disease," he told BBC News.
Alzheimer's disease is the most common form of dementia.
It affects more than 820,000 people in the UK and costs the economy £23bn a year.