Although the healing properties of cannabis have been touted for millennia, research into its potential neuropsychiatric applications truly began to take off in the 1990s following the discovery of the cannabinoid system in the brain. This led to speculation that cannabis could play a therapeutic role in regulating dopamine, serotonin, and other neurotransmitters and offer a new means of treating various ailments.
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At the same time, efforts to liberalize marijuana laws have successfully played out in several nations, including the United States, where, as of April 29, 36 states provide some access to cannabis. These dual tracks – medical and political – have made cannabis an increasingly accepted part of the cultural fabric.
Yet with this development has come a new quandary for clinicians. Medical cannabis has been made widely available to patients and has largely outpaced the clinical evidence, leaving it unclear how and for which indications it should be used.
The many forms of medical cannabis
Cannabis is a genus of plants that includes marijuana (Cannabis sativa) and hemp. These plants contain over 100 compounds, including terpenes, flavonoids, and – most importantly for medicinal applications – cannabinoids.
The most abundant cannabinoid in marijuana is the psychotropic delta-9-tetrahydrocannabinol (THC), which imparts the “high” sensation. The next most abundant cannabinoid is cannabidiol (CBD), which is the nonpsychotropic. THC and CBD are the most extensively studied cannabinoids, together and in isolation. Evidence suggests that other cannabinoids and terpenoids may also hold medical promise and that cannabis’ various compounds can work synergistically to produce a so-called entourage effect.
Patients walking into a typical medical cannabis dispensary will be faced with several plant-derived and synthetic options, which can differ considerably in terms of the ratios and amounts of THC and CBD they contain, as well in how they are consumed (i.e., via smoke, vapor, ingestion, topical administration, or oromucosal spray), all of which can alter their effects. Further complicating matters is the varying level of oversight each state and country has in how and whether they test for and accurately label products’ potency, cannabinoid content, and possible impurities.
Medically authorized, prescription cannabis products go through an official regulatory review process, and indications/contraindications have been established for them. To date, the Food and Drug Administration has approved one cannabis-derived drug product – Epidiolex (purified CBD) – for the treatment of seizures associated with Lennox-Gastaut syndrome or Dravet syndrome in patients aged 2 years and older. The FDA has also approved three synthetic cannabis-related drug products – Marinol, Syndros (or dronabinol, created from synthetic THC), and Cesamet (or nabilone, a synthetic cannabinoid similar to THC) – all of which are indicated for treatment-related nausea and anorexia associated with weight loss in AIDS patients.
Surveys of medical cannabis consumers indicate that most people cannot distinguish between THC and CBD, so the first role that physicians find themselves in when recommending this treatment may be in helping patients navigate the volume of options.
Promising treatment for pain
Chronic pain is the leading reason patients seek out medical cannabis. It is also the indication that most researchers agree has the strongest evidence to support its use.
Dr. Diana Martinez
“In my mind, the most promising immediate use for medical cannabis is with THC for pain,” Diana M. Martinez, MD, a professor of psychiatry at Columbia University, New York, who specializes in addiction research, said in a recent MDedge podcast. “THC could be added to the armamentarium of pain medications that we use today.”
In a 2015 systematic literature review, researchers assessed 28 randomized, controlled trials (RCTs) of the use of cannabinoids for chronic pain. They reported that a variety of formulations resulted in at least a 30% reduction in the odds of pain, compared with placebo. A meta-analysis of five RCTs involving patients with neuropathic pain found a 30% reduction in pain over placebo with inhaled, vaporized cannabis. Varying results have been reported in additional studies for this indication. The National Academies of Sciences, Engineering, and Medicine concluded that there was a substantial body of evidence that cannabis is an effective treatment for chronic pain in adults.
The ongoing opioid epidemic has lent these results additional relevance.
Seeing this firsthand has caused Mark Steven Wallace, MD, a pain management specialist and chair of the division of pain medicine at the University of California San Diego Health, to reconsider offering cannabis to his patients.
Dr. Mark Wallace
“I think it’s probably more efficacious, just from my personal experience, and it’s a much lower risk of abuse and dependence than the opioids,” he said.
Dr. Wallace advised that clinicians who treat pain consider the ratios of cannabinoids.
“This is anecdotal, but we do find that with the combination of the two, CBD reduces the psychoactive effects of the THC. The ratios we use during the daytime range around 20 mg of CBD to 1 mg of THC,” he said.
In a recent secondary analysis of an RCT involving patients with painful diabetic peripheral neuropathy, Dr. Wallace and colleagues showed that THC’s effects appear to reverse themselves at a certain level.
“As the THC level goes up, the pain reduces until you reach about 16 ng/mL; then it starts going in the opposite direction, and pain will start to increase,” he said. “Even recreational cannabis users have reported that they avoid high doses because it’s very aversive. Using cannabis is all about, start low and go slow.”
A mixed bag for neurologic indications
There are relatively limited data on the use of medical cannabis for other neurologic conditions, and results have varied. For uses other than pain management, the evidence that does exist is strongest regarding epilepsy, said Daniel Freedman, DO, assistant professor of neurology at the University of Texas at Austin. He noted “multiple high-quality RCTs showing that pharmaceutical-grade CBD can reduce seizures associated with two particular epilepsy syndromes: Dravet Syndrome and Lennox Gastaut.”
Dr. Daniel Freedman
These findings led to the FDA’s 2018 approval of Epidiolex for these syndromes. In earlier years, interest in CBD for pediatric seizures was largely driven by anecdotal parental reports of its benefits. NASEM’s 2017 overview on medical cannabis found evidence from subsequent RCTs in this indication to be insufficient. Clinicians who prescribe CBD for this indication must be vigilant because it can interact with several commonly used antiepileptic drugs.
Cannabinoid treatments have also shown success in alleviating muscle spasticity resulting from multiple sclerosis, most prominently in the form of nabiximols (Sativex), a standardized oralmucosal spray containing approximately equal quantities of THC and CBD. Nabiximols is approved in Europe but not in the United States. Moderate evidence supports the efficacy of these and other treatments over placebo in reducing muscle spasticity. Patient ratings of its effects tend to be higher than clinician assessment.
Parkinson’s disease has not yet been approved as an indication for treatment with cannabis or cannabinoids, yet a growing body of preclinical data suggests these could influence the dopaminergic system, said Carsten Buhmann, MD, from the department of neurology at the University Medical Center Hamburg-Eppendorf (Germany).
“In general, cannabinoids modulate basal-ganglia function on two levels which are especially relevant in Parkinson’s disease, i.e., the glutamatergic/dopaminergic synaptic neurotransmission and the corticostriatal plasticity,” he said. “Furthermore, activation of the endocannabinoid system might induce neuroprotective effects related to direct receptor-independent mechanisms, activation of anti-inflammatory cascades in glial cells via the cannabinoid receptor type 2, and antiglutamatergic antiexcitotoxic properties.”
Dr. Buhmann said that currently, clinical evidence is scarce, consisting of only four double-blind, placebo-controlled RCTs involving 49 patients. Various cannabinoids and methods of administering treatment were employed. Improvement was only observed in one of these RCTs, which found that the cannabinoid receptor agonist nabilone significantly reduced levodopa-induced dyskinesia for patients with Parkinson’s disease. Subjective data support a beneficial effect. In a nationwide survey of 1,348 respondents conducted by Dr. Buhmann and colleagues, the majority of medical cannabis users reported that it improved their symptoms (54% with oral CBD and 68% with inhaled THC-containing cannabis).
NASEM concluded that there was insufficient evidence to support the efficacy of medical cannabis for other neurologic conditions, including Tourette syndrome, amyotrophic lateral sclerosis, Huntington disease, dystonia, or dementia. A 2020 position statement from the American Academy of Neurology cited the lack of sufficient peer-reviewed research as the reason it could not currently support the use of cannabis for neurologic disorders.
Yet, according to Dr. Freedman, who served as a coauthor of the AAN position statement, this hasn’t stymied research interest in the topic. He’s seen a substantial uptick in studies of CBD over the past 2 years. “The body of evidence grows, but I still see many claims being made without evidence. And no one seems to care about all the negative trials.”