Long-term use of pills for anxiety and sleep problems may be linked to Alzheimer's, research suggests.
A study of older Canadian adults found that past benzodiazepine use for three months or more was linked to an increased risk (up to 51%) of dementia.
NHS guidelines say the drugs should be used for eight to 12 weeks at most.
The French-Canadian team says while the link is not definitive, it is another warning that treatments should not exceed three months.
"Benzodiazepine use is associated with an increased risk of Alzheimer's disease," lead researcher, Sophie Billioti de Gage of the University of Bordeaux, France, and colleagues wrote in the BMJ.
"Unwarranted long-term use of these drugs should be considered as a public health concern."
The study involved about 2,000 cases of Alzheimer's disease in adults aged over 66 living in Quebec. All had been prescribed benzodiazepines.
They were compared with about 7,000 healthy people of the same age living in the same community.
While an increased risk was found in those on benzodiazepines, the nature of the link was unclear.
Dr Eric Karran, director of research at Alzheimer's Research UK, said: "This study shows an apparent link between the use of benzodiazepines and Alzheimer's disease although it's hard to know the underlying reason behind the link.
"One limitation of this study is that benzodiazepines treat symptoms such as anxiety and sleep disturbance, which may also be early indicators of Alzheimer's disease."
Prof Guy Goodwin, president of the European College of Neuropsychopharmacology, said the findings "could mean that the drugs cause the disease, but is more likely to mean that the drugs are being given to people who are already ill".
Dr James Pickett, head of research at the Alzheimer's Society, said with 1.5 million people in the UK being prescribed benzodiazepines at any one time, "evidence that their long-term use increases the risk of dementia is significant, and raises questions about their use".
Benzodiazepines are used to treat anxiety disorders and insomnia.
Despite published guidance on their appropriate use for short-term management, inappropriate prescribing of the drugs is still a concern.
Experts are calling for better monitoring of side-effects, particularly in older adults.