A popular anti-anxiety drug has been linked with an increased risk of dementia in pensioners, according to new research.
Patients over the age of 65 who start taking benzodiazepines, also known as benzos, have a 50% increased chance of developing dementia within 15 years compared with people who had never used the drug, according to the study.
Researchers from the University of Bordeaux, France, warned that "indiscriminate widespread use" of the drugs, which are also used to treat insomnia, should be cautioned against.
The drug is widely used in many countries. In France 30% of people over the age of 65 take benzodiazepines. Many administer the drug for long periods despite guidelines suggesting it should only be used for a few weeks.
The research, which is published on bmj.com, examined 1,063 people with an average age of 78 over two decades. They had never taken the drug before and were all free from dementia.
They found that 95 patients started taking benzodiazepine during the study.
After a 15-year follow-up, 253 people developed dementia. Of these, 30 had begun to take the drugs between three and five years into the study.
The chance of dementia occurring in those who had taken the drugs was 4.8 per 100 "person years" - a statistical measure representing one person at risk of development of a disease during a period of one year.
Of those who had not taken the drugs the likelihood was measured to be 3.2 per 100 person years, the researchers found.
"In this large, prospective, population based study of elderly people who were free of dementia and did not use benzodiazepines until at least the third year of follow-up, new use of benzodiazepines was associated with a significant, approximately 50% increase in the risk of dementia," the authors wrote.
"Benzodiazepines remain useful for the treatment of acute anxiety states and transient insomnia. However, increasing evidence shows that their use may induce adverse outcomes, mainly in elderly people, such as serious falls and fall related fractures.
"Our data add to the accumulating evidence that use of benzodiazepines is associated with increased risk of dementia, which, given the high and often chronic consumption of these drugs in many countries, would constitute a substantial public health concern.
"Therefore, physicians should carefully assess the expected benefits of the use of benzodiazepines in the light of these adverse effects and, whenever possible, limit prescription to a few weeks as recommended by the good practice guidelines.
"In particular, uncontrolled chronic use of benzodiazepines in elderly people should be cautioned against."
Dr Eric Karran, director of research at Alzheimer's Research UK, added: "Long-term population studies are invaluable for helping us to understand how our environment and life events may influence our risk of dementia, but it can be incredibly difficult to tease out the real cause behind these associations.
"It could be the underlying conditions that drive someone to need benzodiazepines, rather than the drugs themselves, that are the important risk factors in this case.
"While more research is needed to understand why benzodiazepines may be associated with an increased risk of dementia, the study does highlight the importance of careful drug prescription."
Alzheimer's Society director of research Professor Clive Ballard added: "This is the not the first time it has been suggested that these drugs could have a negative impact on cognition. With this long-term study adding to the evidence, it emphasises how important it is that we properly monitor how treatments for anxiety or sleep problems are used.
"While Alzheimer's Society is leading the way in calling for an end to the inappropriate use of anti-psychotic drugs for people with dementia, it is also vital that benzodiazepines are not automatically turned to as the alternative. Instead we need to empower care staff with the knowledge they need to understand dementia and the person behind the condition."