In light of the rapidly evolving interest in the potential use of marijuana and its derivative compounds for medical purposes, it is important to take stock of what we know and do not know about the therapeutic potential of CBD.

Background To date, 23 states and the District of Columbia have passed laws allowing marijuana to be used for a variety of medical conditions. Fifteen additional states have enacted laws intended to allow access to CBD oil and/or high-CBD strains of marijuana. Interest in the potential therapeutic effects of CBD has been growing rapidly, partially in response to media attention surrounding the use of CBD oil in young children with intractable seizure disorders including Dravet syndrome and Lennox-Gastaut syndrome. While there are promising preliminary data, the scientific literature is currently insufficient to either prove or disprove the efficacy and safety of CBD in patients with epilepsy.i and further clinical evaluation is warranted. In addition to epilepsy, the therapeutic potential of CBD is currently being explored for a number of indications including anxiety disorders, substance use disorders, schizophrenia, cancer, pain, inflammatory diseases and others. My testimony will provide an overview of what the science tells us about the therapeutic potential of CBD and of the ongoing research supported by NIH in this area. 

CBD Biology and Element X Therapeutic Rationale CBD is one of more than 80 active cannabinoid chemicals in the marijuana plant.ii Unlike the main psychoactive cannabinoid in marijuana, tetrahydrocannabinol (THC), CBD does not produce euphoria or intoxication.iii,iv,v Cannabinoids have their effect mainly by interacting with specific receptors on cells in the brain and body: the CB1 receptor, found on neurons and glial cells in various parts of the brain, and the CB2 receptor, found mainly in the body’s immune system. The euphoric effects of THC are caused by its activation of CB1 receptors. CBD has a very low affinity for these receptors (100 fold less than THC) and when it binds it produces little to no effect. There is also growing evidence that CBD acts on other brain signaling systems, and that these actions may be important contributors to its therapeutic effects.ii Preclinical and Clinical Evidence Rigorous clinical studies are still needed to evaluate the clinical potential of CBD for specific conditions.i However, pre-clinical research (including both cell culture and animal models) has shown CBD to have a range of effects that may be therapeutically useful, including anti-seizure, antioxidant, neuroprotective, anti-inflammatory, analgesic, anti-tumor, anti-psychotic, and anti-anxiety properties. 

Anti-Seizure Effects A number of studies over the last two decades or more have reported that CBD has anti-seizure activity, reducing the severity of seizures in animal models.vi,vii In addition, there have been a number of case studies and anecdotal reports suggesting that CBD may be effective in treating children with drug-resistant epilepsy.viii,ix,x However, there have only been a few small randomized clinical trials examining the efficacy of CBD as a treatment for epilepsy; the total number of subjects enrolled in these studies was 48. Three of the four studies reported positive results, including decreased frequency of seizures. However, the studies suffered from significant design flaws, including failure to fully quantify baseline seizure frequency, inadequate statistical analysis, and a lack of sufficient detail to adequately evaluate and interpret the findings.viii Therefore, the currently available information is insufficient to draw firm conclusions regarding the efficacy of CBD as a treatment for epilepsy; a recent Cochrane review concluded, there is a need for “a series of properly designed, high quality, and adequately powered trials.”xi NIDA is currently collaborating with the National Institute on Neurological Disorders and Stroke to evaluate CBD in animal models of epilepsy in order to understand the underlying mechanisms and optimize the conditions under which CBD may treat seizure disorders, and determine whether it works synergistically with other anti-seizure medications.

In addition, clinical trials are currently underway by GW Pharmaceuticals, testing the efficacy of Epidiolex, a purified CBD extract, for treatment of pediatric epilepsy. Neuroprotective and Anti-Inflammatory Effects CBD has also been shown to have neuroprotective properties in cell cultures as well as in animal models of several neurodegenerative diseases, including Alzheimer’s,xii,xiii,xiv stroke,xv glutamate toxicity,xvi multiple sclerosis (MS),xvii Parkinson’s disease,xviii and neurodegeneration caused by alcohol abuse.xix Nabiximols (trade name Sativex), which contains THC and CBD in roughly equal proportions, has been approved throughout most of Europe and in a number of other countries for the treatment of spasticity associated with MS. It has not been approved in the United States, but clinical trials are ongoing, and two recent studies reported that nabiximols reduced the severity of spasticity in MS patients.xx,xxi There have been limited clinical trials to assess the potential efficacy of CBD for the other indications highlighted; however, a recent small double-blind trial in patients with Parkinson’s disease found the CBD improved quality-of-life scores.xxii Analgesic Effects There have been multiple clinical trials demonstrating the efficacy of nabiximols on central and peripheral neuropathic pain, rheumatoid arthritis, and cancer pain.xxiii In addition, nabiximols is currently approved in Canada for the treatment of central neuropathic pain in MS and cancer pain unresponsive to opioid therapy. 

However, the current evidence suggests that the analgesia is mediated by THC and it is unclear whether CBD contributes to the therapeutic effects.xxiv THC alone has been shown to reduce pain;xxv,xxvi we are unaware of clinical studies that have explored the efficacy of CBD alone on pain. However, the anti-inflammatory properties of CBD (discussed above) could be predicted to play a role in the analgesic effects of nabiximols. Recent research has also suggested that cannabinoids and opioids have different mechanisms for reducing pain and that their effects may be additive, which suggests that combination therapies may be developed that may have reduced risks compared to current opioid therapies. However, this work is very preliminary.xxvii Anti-Tumor Effects In addition to the research on the use of cannabinoids in palliative treatments for cancer—reducing pain and nausea and in increasing appetite—there are also several pre-clinical reports showing anti-tumor effects of CBD in cell culture and in animal models.xxviii These studies have found reduced cell viability, increased cancer cell death, decreased tumor growth, and inhibition of metastasis (reviewed in McAllister et al, 2015).xxix These effects may be due to the antioxidant and anti-inflammatory effects of CBD;xxx however these findings have not yet been explored in human patients. There are multiple industry sponsored clinical trials underway to begin to test the efficacy of CBD in human cancer patients. 

Anti-Psychotic Effects Marijuana can produce acute psychotic episodes at high doses, and several studies have linked marijuana use to increased risk for chronic psychosis in individuals with specific genetic risk factors. Research suggests that these effects are mediated by THC, and it has been suggested that CBD may mitigate these effects.xxxi There have been a few small-scale clinical trials in which patients with psychotic symptoms were treated with CBD, including case reports of patients with schizophrenia that reported conflicting results; a small case study in patients with Parkinson’s disease with psychosis, which reported positive results; and one small randomized clinical trial reporting clinical improvement in patients with schizophrenia treated with CBD.xxxii Large randomized clinical trials would be needed to fully evaluate the therapeutic potential of CBD for patients with schizophrenia and other forms of psychosis. Anti-Anxiety Effects CBD has shown therapeutic efficacy in a range of animal models of anxiety and stress, reducing both behavioral and physiological (e.g., heart rate) measures of stress and anxiety.xxxiii,xxxiv In addition, CBD has shown efficacy in small human laboratory and clinical trials. CBD reduced anxiety in patients with social anxiety subjected to a stressful public speaking task.xxxv In a laboratory protocol designed to model post-traumatic stress disorders, CBD improved “consolidation of extinction learning”, in other words, forgetting of traumatic memories.

The anxiety-reducing effects of CBD appear to be mediated by alterations in serotonin receptor 1a signaling, although the precise mechanism remains to be elucidated and more research is needed.xxxvii Efficacy for Treating Substance Use Disorders Early preclinical findings also suggest that CBD may have therapeutic value as a treatment of substance use disorders. CBD reduced the rewarding effects of morphinexxxviii and reduced cue-induced heroin seekingxxxix in animal models. A few small clinical trials have examined CBD and/or nabiximols (THC/CBD) for the treatment of substance use disorders; however, the available data are not sufficient to draw conclusions. NIDA is supporting multiple ongoing clinical trials in this area. Safety of CBD For reasons discussed previously, despite its molecular similarity to THC, CBD only interacts with cannabinoid receptors weakly at very high doses (100 times that of THC),xl and the alterations in thinking and perception caused by THC are not observed with CBD.iii.iv,v The different pharmacological properties of CBD give it a different safety profile from THC. A review of 25 studies on the safety and efficacy of CBD did not identify significant side effects across a wide range of dosages, including acute and chronic dose regimens, using various modes of administration.xli CBD is present in nabiximols which, as noted earlier, is approved throughout most of Europe and in other countries. Because of this, there is extensive information available with regard to its metabolism, toxicology, and safety. However, additional safety testing among specific patient populations may be warranted should an application be made to the Food and Drug Administration. Research Opportunities and Challenges This is a critical area for new research.

The primary sensory subtest for the trigeminal system is sensory discrimination.

A cotton-tipped applicator, which is cotton attached to the end of a thin wooden stick, can be used easily for this. The wood of the applicator can be snapped so that a pointed end is opposite the soft cotton-tipped end. The cotton end provides a touch stimulus, while the pointed end provides a painful, or sharp, stimulus. While the patient’s eyes are closed, the examiner touches the two ends of the applicator to the patient’s face, alternating randomly between them. The patient must identify whether the stimulus is sharp or dull. These stimuli are processed by the trigeminal system separately. Contact with the cotton tip of the applicator is a light touch, relayed by the chief nucleus, but contact with the pointed end of the applicator is a painful stimulus relayed by the spinal trigeminal nucleus. Failure to discriminate these stimuli can localize problems within the brain stem. If a patient cannot recognize a painful stimulus, that might indicate damage to the spinal trigeminal nucleus in the medulla. The medulla also contains important regions that regulate the cardiovascular, respiratory, and digestive systems, as well as being the pathway for ascending and descending tracts between the brain and spinal cord. 

Damage, such as a stroke, that results in changes in Focused In sensory discrimination may indicate these unrelated regions are affected as well. Gaze Control The three nerves that control the extraocular muscles are the oculomotor, trochlear, and abducens nerves, which are the third, fourth, and sixth cranial nerves. As the name suggests, the abducens nerve is responsible for abducting the eye, which it controls through contraction of the lateral rectus muscle. The trochlear nerve controls the superior oblique muscle to rotate the eye along its axis in the orbit medially, which is called intorsion, and is a component of focusing the eyes on an object close to the face. The oculomotor nerve controls all the other extraocular muscles, as well as a muscle of the upper eyelid. Movements of the two eyes need to be coordinated to locate and track visual stimuli accurately. When moving the eyes to locate an object in the horizontal plane, or to track movement horizontally in the visual field, the lateral rectus muscle of one eye and medial rectus muscle of the other eye are both active. The lateral rectus is controlled by neurons of the abducens nucleus in the superior medulla, whereas the medial rectus is controlled by neurons in the oculomotor nucleus of the midbrain. Coordinated movement of both eyes through different nuclei requires integrated processing through the brain stem. 

In the midbrain, the superior colliculus integrates visual stimuli with motor responses to initiate eye movements. The paramedian pontine reticular formation (PPRF) will initiate a rapid eye movement, or saccade, to bring the eyes to bear on a visual stimulus quickly. These areas are connected to the oculomotor, trochlear, and abducens nuclei by the medial longitudinal fasciculus (MLF) that runs through the majority of the brain stem. The MLF allows for conjugate gaze, or the movement of the eyes in the same direction, during horizontal movements that require the lateral and medial rectus muscles. Control of conjugate gaze strictly in the vertical direction is contained within the oculomotor complex. To elevate the eyes, the oculomotor nerve on either side stimulates the contraction of both superior rectus muscles; to depress the eyes, the oculomotor nerve on either side stimulates the contraction of both inferior rectus muscles. Purely vertical movements of the eyes are not very common. Movements are often at an angle, so some horizontal components are necessary, adding the medial and lateral rectus muscles to the movement. The rapid movement of the eyes used to locate and direct the fovea onto visual stimuli is called a saccade. Notice that the paths that are traced in [link] are not strictly vertical. The movements between the nose and the mouth are closest, but still have a slant to them. Also, the superior and inferior rectus muscles are not perfectly oriented with the line of sight. 

The origin for both muscles is medial to their insertions, so elevation and depression may require the lateral rectus muscles to compensate for the slight adduction inherent in the contraction of those muscles, requiring MLF activity as well. The left panel of this figure shows a painting of a woman’s face, and the right panel shows lines traced over the painting. These lines represent the shifts in the gaze of a person looking at another face. Saccades are rapid, conjugate movements of the eyes to survey a complicated visual stimulus, or to follow a moving visual stimulus. This image represents the shifts in gaze typical of a person studying a face. Notice the concentration of gaze on the major features of the face and the large number of paths traced between the eyes or around the mouth. Testing eye movement is simply a matter of having the patient track the tip of a pen as it is passed through the visual field. This may appear similar to testing visual field deficits related to the optic nerve, but the difference is that the patient is asked to not move the eyes while the examiner moves a stimulus into the peripheral visual field. Here, the extent of movement is the point of the test. The examiner is watching for conjugate movements representing proper function of the related nuclei and the MLF. Failure of one eye to abduct while the other adducts in a horizontal movement is referred to as internuclear ophthalmoplegia. When this occurs, the patient will experience diplopia, or double vision, as the two eyes are temporarily pointed at different stimuli. 

Diplopia is not restricted to failure of the lateral rectus, because any of the extraocular muscles may fail to move one eye in perfect conjugation with the other. The final aspect of testing eye movements is to move the tip of the pen in toward the patient’s face. As visual stimuli move closer to the face, the two medial recti muscles cause the eyes to move in the one nonconjugate movement that is part of gaze control. When the two eyes move to look at something closer to the face, they both adduct, which is referred to as convergence. To keep the stimulus in focus, the eye also needs to change the shape of the lens, which is controlled through the parasympathetic fibers of the oculomotor nerve. The change in focal power of the eye is referred to as accommodation. Accommodation ability changes with age; focusing on nearer objects, such as the written text of a book or on a computer screen, may require corrective lenses later in life. Coordination of the skeletal muscles for convergence and coordination of the smooth muscles of the ciliary body for accommodation are referred to as the accommodation–convergence reflex. A crucial function of the cranial nerves is to keep visual stimuli centered on the fovea of the retina. The vestibulo-ocular reflex (VOR) coordinates all of the components ([link]), both sensory and motor, that make this possible. If the head rotates in one direction—for example, to the right—the horizontal pair of semicircular canals in the inner ear indicate the movement by increased activity on the right and decreased activity on the left. 

The information is sent to the abducens nuclei and oculomotor nuclei on either side to coordinate the lateral and medial rectus muscles. The left lateral rectus and right medial rectus muscles will contract, rotating the eyes in the opposite direction of the head, while nuclei controlling the right lateral rectus and left medial rectus muscles will be inhibited to reduce antagonism of the contracting muscles. These actions stabilize the visual field by compensating for the head rotation with opposite rotation of the eyes in the orbits. Deficits in the VOR may be related to vestibular damage, such as in Ménière’s disease, or from dorsal brain stem damage that would affect the eye movement nuclei or their connections through the MLF. If the head is turned in one direction, the coordination of that movement with the fixation of the eyes on a visual stimulus involves a circuit that ties the vestibular sense with the eye movement nuclei through the MLF. An iconic part of a doctor’s visit is the inspection of the oral cavity and pharynx, suggested by the directive to “open your mouth and say ‘ah.’” This is followed by inspection, with the aid of a tongue depressor, of the back of the mouth, or the opening of the oral cavity into the pharynx known as the fauces. Whereas this portion of a medical exam inspects for signs of infection, such as in tonsillitis, it is also the means to test the functions of the cranial nerves that are associated with the oral cavity. 

The facial and glossopharyngeal nerves convey gustatory stimulation to the brain. Testing this is as simple as introducing salty, sour, bitter, or sweet stimuli to either side of the tongue. The patient should respond to the taste stimulus before retracting the tongue into the mouth. Stimuli applied to specific locations on the tongue will dissolve into the saliva and may stimulate taste buds connected to either the left or right of the nerves, masking any lateral deficits. Along with taste, the glossopharyngeal nerve relays general sensations from the pharyngeal walls. These sensations, along with certain taste stimuli, can stimulate the gag reflex. If the examiner moves the tongue depressor to contact the lateral wall of the fauces, this should elicit the gag reflex. Stimulation of either side of the fauces should elicit an equivalent response. The motor response, through contraction of the muscles of the pharynx, is mediated through the vagus nerve. Normally, the vagus nerve is considered autonomic in nature. The vagus nerve directly stimulates the contraction of skeletal muscles in the pharynx and larynx to contribute to the swallowing and speech functions. Further testing of vagus motor function has the patient repeating consonant sounds that require movement of the muscles around the fauces. The patient is asked to say “lah-kah-pah” or a similar set of alternating sounds while the examiner observes the movements of the soft palate and arches between the palate and tongue. The facial and glossopharyngeal nerves are also responsible for the initiation of salivation.