A new question must be added to the list that mental health clinicians ask their older patients while taking a history, regardless of age: “Have you used any cannabis products recently?”

I learned this lesson during a follow-up appointment with an 82-year-old man with Alzheimer disease. He reported some rather abrupt cognitive decline, and I asked him and his wife about recent medical changes without finding any clear precipitant. Finally, his wife sheepishly confessed that their son had sent them a grab bag of cannabis-derived products that the patient was taking. The family was desperate for anything to help his cognitive decline and they had heard that cannabis was a miracle cure for Alzheimer disease and many other conditions. Unfortunately, the patient received no benefit but did experience some mild, noticeable adverse effects. Without my probing, I never would have known. The next patient I asked explicitly about using cannabis products responded in an equally explicit manner: “No I haven’t,” he said, “but can you get me some?” It was clear that the age of cannabis had arrived.

Figure. Percentage of Individuals 65 or Older Who Have Used Medical Marijuana

Older individuals with psychiatric disorders are increasingly using cannabis, largely in the form of prescribed medical marijuana and cannabidiol (CBD). This trend has been driven by several factors: reduced stigma, lifted restrictions on possession and sale by most states, and the enormous proliferation of articles and ads that tout supposed benefits of cannabis for many conditions that afflict the elderly, including chronic pain, peripheral neuropathy, stress, anxiety, depression, insomnia, headaches, and the adverse effects of chemotherapy.^1,2 As a result, medical marijuana use by individuals 65 or older has increased more than 8-fold in recent years, from less than 0.5% in 2006 to 4.2% in 2018³ (Figure). In a survey of 345 adults in Colorado, 16% had used medical marijuana since legalization, with half the users being 75 or older.⁴ CBD has seen a particularly enormous surge in interest and availability after it was exempted from federal regulations in the 2018 US Farm Bill. Since then, it is estimated that 6.4% of adults aged 45 to 55 and 3.7% of those 55 or older have used CBD at least once.⁵

Cannabis, Cognition, and Quality of Life

Although the cannabis sativa plant contains hundreds of chemicals known as cannabinoids, the 2 main active chemicals are Δ-٩-tetrahydrocannabinol (THC) and CBD. THC has both psychotropic and euphoric properties and confers the high associated with marijuana, while CBD is noneuphoric because it does not activate the cannabinoid 1 receptor like THC does. The same plants that produce THC also produce CBD. Both THC and CBD come in a variety of forms; they can be smoked, vaped, eaten, ingested as liquids, used as oils, and applied via creams and cosmetics.

The psychotropic effects of both THC and CBD are mainly because of their interaction with endogenous cannabinoid receptors in the brain, as well as activation of a variety of other cannabinoid and noncannabinoid receptors (ie, serotonergic, glutaminergic, µ-opioid, and α1-adrenergic) in the central nervous system and on immunologic cells. Based on their known and theorized properties, marijuana and CBD have been proposed as remedies to modulate symptoms of stress, insomnia, pain, inflammation, and cognitive impairment, particularly in older individuals. As a result, there are many unsubstantiated claims that THC and/or CBD can treat Alzheimer disease, Parkinson disease, schizophrenia, and anxiety and mood disorders.⁶

Several small case series have suggested a tentative basis for some of these claims. For example, a study looked at the use of an oral spray containing THC and CBD for peripheral nerve pain and found that it was well tolerated and showed a trend toward improvement in pain and sleep.⁷ Medical marijuana use has been associated with some improvement in both movements and neuropsychiatric symptoms in individuals with Parkinson disease.⁶ One small case series found improvement in behavioral and psychological symptoms of dementia from cannabis oil containing THC.⁸ A related randomized control trial of 50 individuals with dementia and associated neuropsychiatric symptoms did not see any benefit from THC, but did find it was well tolerated and with no appreciable effect on memory.⁹ Despite these limited data, agitation associated with dementia is a qualifying condition for medical marijuana in several states.¹⁰

Gruber and colleagues studied a sample of middle-aged individuals using medical marijuana over 3 months and found improvements in executive function and reductions in depression, insomnia, and impulsivity.^1,11 They also saw a reduction in the use of benzodiazepines and opioids as well as a normalization of brain activation waves to states seen in normal controls. It is not clear, however, if these neurocognitive results were the direct result of the medical marijuana or an indirect benefit of decreases in pain, anxiety, and insomnia.

Although there are many other in vivo, in vitro, and animal studies of THC and CBD, it is clear that evidence is limited, and there are few randomized controlled trials to support most of their theorized clinical benefits. In addition, these studies are based on a variety of cannabis products from multiple sources and with variable contents.¹² The 2 exceptions include a Food and Drug Administration (FDA)-approved form of synthetic THC known as dronabinol, which is used for treating appetite and weight loss resulting from AIDS as well as nausea and vomiting from chemotherapy, and a medication containing CBD for the treatment of 2 rare childhood seizure disorders. Dronabinol has also been studied in Alzheimer disease to treat anorexia and agitation and was found to be well tolerated with variable efficacy for both conditions.¹³ In the European Union, nabiximols, a CBD-THC combination, has been approved for the treatment of muscle spasms associated with multiple sclerosis.