When Jane Simpson’s mother, then 91, started showing signs of memory loss in December 2007, Ms. Simpson thought age had finally caught up with her. “As this had been a gradual process, and considering her age, we were not unduly alarmed — just saddened that it seemed we were losing my mother mentally,” she wrote in an e-mail to this blog.

But on a visit six months later, Ms. Simpson, a 61-year-old advertising copywriter in North Carolina, was struck by how much worse her mother’s memory loss had become and by her confusion about everything happening around her.

Just typical 91-year-old behavior? Just the first signs of the inevitable slide toward dementia we all may face if we live long enough? Not at all.

Since the ’70s, geriatric specialists have been aware of many unusual causes of memory loss, confusion and disorientation in older people. These include not just medical conditions ranging from urinary tract infection to hydrocephalus to the flu, but also side effects from many commonly used medications.

Often, doctors and family members disregard these symptoms, thinking that they are just signs of an inevitable age-related decline. But many cases of pseudo-senility, as it’s called, are reversible — if they are caught early enough.

By coincidence, Ms. Simpson had recently read a short article in her local newspaper about the side effects in the elderly of a bladder control drug called Ditropan, which include severe memory loss. Her mother was taking Ditropan.

Ms. Simpson and her sister got their mother switched to an alternative bladder control drug, Enablex. Sure enough, her mental symptoms eased. “Within three months,” Ms. Simpson recalled in her e-mail, “we felt that we had our mother back.”

You’d think that with the constant attention being paid to Alzheimer’s disease and other causes of memory loss in aging, doctors would have learned by now that severe memory loss in healthy aging is a problem to be diagnosed, not an inevitability to be lived with and accepted.

But according to the National Institute on Aging, missed diagnoses of reversible dementia still occur too often. “Some physical and mental changes occur with age in healthy people,” the agency writes in a publication called “Forgetfulness.” “However, much pain and suffering can be avoided if older people, their families, and their doctors recognize dementia as a disease, not part of normal aging.”

Reversible causes of dementia include the side effects of many medications. Ditropan is only the most recent addition to the list, occasioned in part by a study by U.S. Navy neurologist Dr. Jack Tsao involving 870 men and women with an average age of 75. The subjects, who were all Catholic priests, nuns and brothers, were followed for almost eight years. At a 2008 meeting of the American Academy of Neurology, Dr. Tsao reported that those who were taking anticholinergic drugs — a class that includes not only Detrol but also drugs to treat hypertension, asthma and Parkinson’s disease — had a 50 percent higher rate of cognitive decline than those who were not.

According to Dr. Samuel Gandy, a neurologist at Mt. Sinai School of Medicine in Manhattan, drugs with antihistamines often cause mental confusion and sedation in the elderly — especially those containing the antihistamine doxylamine, such as the sleep aid Unisom. In addition, confusion and forgetfulness in the elderly can be caused by malnutrition, chronic alcoholism and metabolic disturbances such as thyroid, kidney or liver disorders — and even on occasion by something as common as dehydration or a high fever.

Doctors are better than they used to be at diagnosing pseudosenility, according to Dr. Gandy. “Most physicians are taught in medical schools to evaluate a patient for dementia by first excluding the reversible causes,” he said.

But one kind remains a particular problem, he added: depression masquerading as dementia. Doctors can’t simply order a test to rule it out; the best they can do is recommend a trial run of antidepressant therapy, he said, “to see if the person will perk up and come back.”

First, though, the physician must think of something other than aging as a possible cause of the symptoms.