The list of drugs that can dull the senses includes some you would expect, such as opiate-based pain relievers, as well as some surprises-certain antibiotics, for example. Even over-the-counter drugs, notably older antihistamines such as chlorpheniramine (Chlor-Trimeton and generic) and diphenhydramine (Benadryl Allergy and generic), can cloud thinking.

Many of the worst offenders belong to a broad class of drugs called anticholinergics, which are used to treat asthma, stomach ulcers, urinary incontinence, and other conditions. In an eight-year study of nearly 400 older people, researchers found that those taking anticholinergics were no more likely than nonusers to develop dementia, but did face a higher risk of impaired cognitive function. The researchers estimate that about 20 percent of cognitive impairment in older people is attributable to anticholinergics.

If you suspect that a medication is affecting your thinking, talk to your doctor about reducing the dose or substituting another drug.