More older adults are using marijuana for sleep, anxiety and pain. A small but growing number are taking it to manage their dementia symptoms .

Doctors who prescribe cannabis to dementia patients say it can alleviate anxiety, agitation and pain, and improve sleep , appetite and mood. While there isn’t much definitive research on the use of cannabis for dementia, several small studies have backed its usefulness in soothing agitation.

Roughly 20% of the people who come for a cannabis consultation now are dementia patients, says Dr. Jeffrey Hergenrather, a general practitioner in Sebastopol, Calif., who has made cannabis his specialty for more than 25 years. Some dementia patients —and their caregivers—are seeking alternatives after traditional medications haven’t provided relief or caused unsettling side effects, doctors say.

“It’s a fairly common practice among people living with dementia,” says Brenda Roberts, executive director of the nonprofit National Council of Dementia Minds, a dementia-patient advocacy group . Marijuana comes up frequently in discussion groups, she adds.

She says her 72-year-old husband, Mark Roberts, who was diagnosed with vascular dementia a little over 10 years ago, takes a liquid dose of cannabis twice a day to help control anxiety and outbursts. He takes a higher-dose THC gummy at night as needed to help with sleep.

Mark Roberts says cannabis helps him relax—and improves his relationship with his wife. The couple live in Elwell, Mich.

“It just calms me down,” he says. “It gets your nerves down so you’re not so rattled. It helps your thought process so you’re not so upset.”

Questions and risks

Cannabis can lead to drowsiness, which raises the serious risk of falls in older people with dementia. That drowsiness can further weaken cognitive functioning in Alzheimer’s patients, says Dr. Jacobo Mintzer, a psychiatrist and professor at the Medical University of South Carolina who is researching cannabis treatment in dementia patients.

Most doctors who support cannabis for dementia currently recommend it for people in the middle to late stages of the disease. They say further study is needed to see if it can be used in earlier stages, and to weigh benefits with potential risks.

They typically prescribe THC, the main psychoactive component of marijuana, in combination with CBD, another part of the cannabis plant that doesn’t produce a high. CBD can decrease the psychoactive effects of THC and enhance the positive properties, Mintzer says.

A new National Institute on Aging study is evaluating a combination of CBD and THC in late-stage hospice-eligible dementia patients who suffer from severe agitation. Typically, they are treated with antipsychotics. But those have side effects including severe constipation, confusion and sometimes even increased agitation, says Mintzer, who is principal investigator of the study.

“We expect that this treatment will allow patients to die with dignity and without agitation or fear,” he says. “The caregivers are delighted that we’re trying new medications to improve the quality of life in the last weeks or days of the patients’ lives.”

Anica Leon-Weil says her mother, Barbara Leon, who has Alzheimer’s disease and lives in an assisted-living care community in Santa Cruz, Calif., has been taking THC edibles for about 10 months. Leon-Weil thought of edibles for her mother after noticing that she was experiencing more anxiety and psychosis symptoms while on other medication.

At first, her mother took an edible during the day to see if it would help with anxiety, says Leon-Weil, a mental health therapist. But her mother seemed less steady on her feet, so she switched to taking it only at night.

The edible has “been really helpful with sleep in particular and to some degree anxiety,” Leon-Weil says. “She is much more able to stay asleep through the night and just kind of relax in her bed even if she wakes up.”

What the science says

There isn’t much definitive research on cannabis and its impact on cognitive impairment and dementia, says Dr. Ronald Petersen, a professor of neurology at Mayo Clinic College of Medicine and Science in Rochester, Minn. Still, he has had patients and caregivers ask about it and hasn’t discouraged them from taking it.

“Sometimes it can produce a calming effect for people,” he says, noting that it isn’t clear whether it does anything to treat the underlying disease.

A recent study of 75 Alzheimer’s patients indicated, in preliminary results , that 5 milligrams of synthetic THC twice a day significantly decreased agitation, compared with the placebo. The researchers, from Johns Hopkins University School of Medicine and Tufts University School of Medicine, presented the findings at several conferences this year and are preparing them for publication. A 2019 study had similar findings, but also reported increased drowsiness among some people.

Surprisingly, THC appeared to improve cognitive function in older animals in some studies, says Ziva Cooper, director of the UCLA Center for Cannabis and Cannabinoids, even though it impairs functioning in younger animals.

Cooper has received National Institutes of Health funds to compare the effects of cannabis in groups of younger, middle-age and older adults.

“There really hasn’t been comparisons,” she says. “That is a serious gap given that so many older people are starting to use it.”

Write to Sumathi Reddy at Sumathi.Reddy@wsj.com