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However, that doesn't mean the approach is a failure, the investigators report in the May 13 online edition of Neurology.
The researchers say the medical marijuana pills were well-tolerated, so it seems safe to test a higher dose in future studies.
The lack of side effects suggests the dose was too low, according to Dr. Marcel Olde Rikkert and his colleagues at Radboud University Medical Center, in the Netherlands.
In the United States, more than 5 million people have Alzheimer's disease, the most common form of dementia, according to the Alzheimer's Association. That number is expected to swell to more than 7 million in the next decade.
Besides memory loss, most patients with dementia also develop behavioral and psychiatric problems, said Dr. Norman Relkin, an associate professor of clinical neuroscience at Weill Cornell Medical College in New York City.
"Right now, there are no medications specifically approved for these types of symptoms," said Relkin, a spokesperson for the American Federation for Aging Research.
"The medications we use are all 'borrowed' from the armamentarium for other conditions that are similar to these dementia symptoms," Relkin explained. So people with dementia may take antidepressants or anti-anxiety drugs, for example.
But those medications have limited effectiveness and can cause side effects.
"We need better interventions," Relkin said. "This is a major unmet need."
Relkin, who was not involved in the new study, said there were early hints that THC -- the substance responsible for marijuana's "high" -- could help. Animal research suggested it might protect brain cells from Alzheimer's-like damage, and one study of dementia patients found that a pill version of THC seemed to ease sleep disturbances.
In this latest research, though, THC pills looked no better than placebo (inactive) pills.
The study included 50 seniors with dementia. About half lived at home; the others were in care facilities.
Olde Rikkert's team randomly assigned them to take either 1.5 mg of THC or a placebo pill three times daily, for three weeks.
The researchers focused on two types of behavioral symptoms: agitation and aggression; and wandering. They found that over three weeks, patients in both groups showed a small improvement, on average.
But the medical marijuana group was no better off than the placebo group, the investigators said.
"The improvement in the placebo group was remarkable, as dementia is a progressive disease," noted Olde Rikkert, who is chair of geriatrics at Radboud Alzheimer Center. "The improvement might have been caused by the fact that the patients received a lot of support during the study, or to the placebo effect."
The other positive news, said the researchers, is that the THC pills seemed well-tolerated. They found no effects on memory or heart rate, and potential side effects such as sleepiness and dizziness were no more common in the medical marijuana group than the placebo group.
Because of that, they say studies testing a higher THC dose are warranted.
"We currently plan to do another study in the same patient group using a higher dose of THC, although the exact dose has not yet been determined," Olde Rikkert said.
At a high dose, however, "there could be negative effects on behavior or cognition," Relkin said.
To Relkin, the findings highlight a bigger issue. "When (existing) drugs are tested in dementia patients, they just don't work the same as they do in other contexts."
The same may be true with medical marijuana, Relkin said.
"The fact that something has a calming effect on a person without dementia does not necessarily mean it's the good, or compassionate, thing to do for a person with dementia," he said. "The brain is very much altered in dementia."
There are non-drug options for addressing dementia patients' behavioral issues, Relkin said. They center on changing the environment: In a nursing home, for example, exit doors can be painted the same color as the walls, so it's more difficult for patients to wander away.
The room doors, Relkin said, can be painted different colors, to help residents find their room and avoid becoming disoriented.
At home, families can take steps, too, he said. If a dementia patient sees a home health aide as a stranger invading the house, for example, families can reintroduce the aide every morning.
"It's important to try to see things through the eyes of the person with dementia," Relkin said.
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