Meta-analyses show that cannabis seems to have a small positive effect on pain (Allan et al. 2018) and spasticity management (Allan et al. 2018; Whiting et al. 2015), as well as cancer-induced nausea and vomiting, but has no effects on cancer-related pain (Allan et al. 2018). For anxiety (NAS 2017; Whiting et al. 2015), sleep quality, and sleep conditions (Whiting et al. 2015), there is not enough evidence to do a meta-analysis nor to adequately report on the efficacy of cannabis. Most studies included in meta-analyses on cannabis in the non-SCI literature are of low quality, mostly due to small group sizes and varying types of cannabinoids, modes of administration and dosage used (Whiting et al. 2015). This makes the scientific value of this evidence low. Some guideline recommendations show that cannabinoids produce analgesia in central neuropathic pain states. There are several studies where cannabis is used for management of HIV neuropathy, post trauma or post surgery, allodynia, as well as combinations of central and peripheral neuropathic pain (Moulin et al. 2014).
Mücke et al. (2018) conducted a meta-analysis that included 1,750 participants, of which 712 were people with Multiple Sclerosis (MS) and 1,038 were people with central or peripheral pain or with other aetiologies, including diabetic polyneuropathy, plexus neuropathy, or unknown/mixed aetiologies. Small positive effects were found for neuropathic pain reduction (Mücke et al. 2018) and inhaled cannabis resulted in short-term reductions in chronic neuropathic pain for 1 in every 5 to patients treated (Andreae et al. 2015). Allan et al. (2018) meta-analysis described a small positive effect on central pain and chronic pain reduction in a subgroup of 298 patients with MS from seven RCTs. A third meta-analysis on cancer-related pain showed no benefit of cannabis therapy when added to ‘care-as-usual’ for individuals with advanced cancer (Boland et al. 2020).
The combined effects of studies predominantly done on multiple sclerosis (MS) patients found that cannabis improved self-reported spasticity in 50% of patients; compared to 35% of patients taking a placebo pill (Allan et al. 2018; Whiting et al. 2015).
Nausea and Vomiting
In a sample of patients on chemotherapy (N=1,215) or in palliative care (N=307), 47% of participants using cannabis, compared to 13% taking a placebo described control of nausea and vomiting (Allan et al. 2018). When we compare cannabis to medications frequently prescribed for nausea (neuroleptics) the effect is a little less convincing, but still substantial: 31% of cannabis patients were able to control nausea and vomiting versus 16% of patients for the other medications. However, patients on cannabis preferred cannabis over the other medications. Whiting et al. (2015) showed that the average number of patients who show complete resolution of nausea and vomiting response is greater in cannabinoids than placebo, but quality of the included studies was low due to small sample size, especially for the trial of glaucoma (N=6), Tourette syndrome (average N=18), sleep disorder (average N=27), and anxiety disorder (N=24) (Whiting et al. 2015).
A small study (N=24) showed some effect of cannabis on people with generalized anxiety disorder compared to placebo for a simulated public speaking trial (Bergamaschi et al. 2011). Additional trials show effects on non-specified anxiety symptoms in non-anxiety disorder patients, but effects were limited (NAS 2017; Whiting et al. 2015). Anxiety symptom outcomes in people with chronic pain suggest a greater effect of cannabinoids than the placebo (Whiting et al. 2015).
There are some positive effects of cannabis (containing both THC and CBD) on sleep problems like insomnia, sleep apnea and sleep restlessness (Whiting et al. 2015). It also seems to improve sleep quality and restfulness in patients without sleep issues, though this effect has only been measured short term (Whiting et al. 2015). The main issue with the studies on sleep, in addition to their small effects, is that they are short in duration; it is expected that tolerance and then dependence will develop. In individuals using larger amounts of cannabis for a longer time, a rebound effect is found on sleep; causing insomnia after ceasing cannabis use (Babson et al. 2017). The effects on sleep of products containing only CBD have not been sufficiently studied yet, but laboratory studies suggest that CBD has a stimulating effect in low doses and a sedating effect in high doses; low dose CBD may increase total sleep time and decrease frequency of awakenings during the night (Carlini & Cunha 1981).
The usefulness of these findings is limited by sample size, variations across studies in cannabis products, cannabis production methods, properties of cannabinoid types and ratios, dosage regimens, and methods of administration. For example, comparing a study of 7% vaporized synthetic THC with a study that used a plant-derived 1:1 THC to CBD ratio cannabis edible is difficult and leaves us guessing about the ideal or preferred cannabinoid medication. Lastly, it is important to note that the effects measured in these studies were small and the clinical relevance falls within a grey area; therefore, it will be up to the clinician to determine what is clinically relevant versus what has been established experimentally.