If you are used to taking commonly prescribed antidepressants, think twice. According to a study, you may be at high risk of developing dementia, even 20 years before the actual diagnosis.

The researchers found a greater incidence of dementia among patients prescribed anticholinergic antidepressants, anticholinergic bladder medications as well as anticholinergic Parkinson's disease medications than among older adults who were not prescribed these drugs.

"Anticholinergics, medications that block acetylcholine, a nervous system neurotransmitter, have previously been implicated as a potential cause of cognitive impairment," said Noll Campbell, assistant professor at the Purdue University in Indiana.

However, the researchers are yet not aware of "exactly how anticholinergics might cause dementia", the researchers said.

For the study, published in the journal BMJ, an international team from the US, UK and Ireland analysed more than 27 million prescriptions as recorded in the medical records of 40,770 patients over age 65 diagnosed with dementia compared to the records of 283,933 older adults without dementia.

"The findings make it clear that clinicians need to carefully consider the anticholinergic burden of their patients and weigh other options," said Malaz Boustani, from the US-based Regenstrief Institute.

"Physicians should review all the anticholinergic medications - including over-the-counter drugs - that patients of all ages are taking and determine safe ways to take individuals off anticholinergic medications in the interest of preserving brain health," Boustani said.