Many medications - including some popular over the counter drugs - have strong anticholinergic effects.
Anticholinergics are a class of drug that blocks the action of the neurotransmitter acetylcholine in the brain and body.
This can lead to many side effects, including drowsiness, constipation, retaining urine and dry mouth and eyes.
The researchers, led by Shelly Gray, a professor in the University of Washington School of Pharmacy in Seattle, reported their findings in JAMA Internal Medicine in January 2015.
Prof. Gray says:
"Older adults should be aware that many medications - including some available without a prescription, such as over-the-counter sleep aids - have strong anticholinergic effects."
People should not stop their therapy but talk to their health care provider
Prof. Gray urges people not to stop their therapy based on the findings of this study - they should talk to their health care provider, and also tell them about all their over-the-counter drug use.
"Health care providers should regularly review their older patients' drug regimens - including over-the-counter medications - to look for chances to use fewer anticholinergic medications at lower doses," she says.
If providers need to prescribe anticholinergics to their patients because they offer the best treatment, then "they should use the lowest effective dose, monitor the therapy regularly to ensure it's working, and stop the therapy if it's ineffective," she adds.
Although the link between raised risk of dementia and anticholinergics has been found before, the new study uses more rigorous methods - including over 7 years of follow-up - to establish the strength of the link. By accessing pharmacy records, the researchers were also able to include non-prescription use of anticholinergics in their data.
It is also the first study to show a dose-response effect, note the authors. That is, the higher the cumulative amount of drug taken, the higher the risk of developing dementia.
And another first for the study, is that it also shows that dementia risk linked to anticholinergics may persist long after people stop taking the drugs.
Taking anticholinergics for more than 3 years linked to higher dementia risk
For their study, Prof. Gray and colleagues tracked nearly 3,500 men and women aged 65 and over with no dementia symptoms at the start of the study. The participants were part of the Adult Changes in Thought (ACT) study in Group Health, an integrated health care delivery system in Seattle.
To assess how much exposure the participants had to anticholinergic drugs, the researchers used computer records from the pharmacies that dispensed them.
From the pharmacy data they added up all the standard daily doses and worked out the cumulative anticholinergic exposure for each participant over the past 10 years. This was updated as participants were followed up for an average of 7 years.
Over the period of the study, nearly 800 participants developed dementia.
The results showed that the most commonly used medications were tricyclic antidepressants (for example, doxepin or Sinequan), first-generation antihistamines (chlorpheniramine, Chlor-Trimeton), and antimuscarinics for bladder control (oxybutynin, Ditropan).
The researchers estimated that people taking at least 10 mg per day of doxepin, 4 mg per day of chlorpheniramine, or 5 mg per day of oxybutynin for more than 3 years would be at greater risk for developing dementia.
Prof. Gray also told Medical News Today the dose risks associated with Benadryl: "The dose of diphenhydramine that would correspond to the highest risk group is taking the equivalent of 50 mg each day for longer than 3 years - or 25 mg per day for longer than 6 years in duration."
Prof. Gray says there are alternative non-anticholinergic drugs for doxepin and chlorpheniramine. For example, to treat depression there are the selective serotonin re-uptake inhibitor (SSRI) like citalopram (Celexa) or fluoxetine (Prozac). And there are second generation antihistamines like loratadine (Claritin) for allergies.
However, while there are not many alternatives to oxybutynin for increasing bladder control, she suggests behavioral changes may be an option.
Some of the ACT participants have agreed to have their brains autopsied after they die. This might reveal if taking anticholinergic medications is more likely to result in brain changes that are characteristic of people who develop Alzheimer's disease.
Funds for the study came from the NIH's National Institute on Aging and the Branta Foundation.
Meanwhile, Medical News Today recently learned how a team of scientists and engineers are developing an MRI method for non-invasive, early detection of Alzheimer's disease. They have so far shown that it works in animals. The new brain imaging method detects the toxin that leads to Alzheimer's disease before typical symptoms appear.