We report a systematic review and meta-analysis of studies which assessed the antinociceptive efficacy of cannabinoids, cannabis-based medicines, and endocannabinoid system modulators on pain-associated behavioural outcomes in animal models of pathological or injury-related persistent pain. In April 2019, we systematically searched 3 online databases and used crowd science and machine learning to identify studies for inclusion. We calculated a standardised mean difference (SMD) effect size for each comparison and performed a random effects meta-analysis. We assessed the impact of study design characteristics and reporting of mitigations to reduce the risk of bias. We meta-analysed 374 studies in which 171 interventions were assessed for antinociceptive efficacy in rodent models of pathological or injury-related pain. Most experiments were conducted in male animals (86 %). Antinociceptive efficacy was most frequently measured by attenuation of hypersensitivity to evoked limb withdrawal. Selective CB₁, CB₂, non-selective cannabinoid receptor agonists (including delta-9-tetrahydrocannabinol; THC), and PPAR-alpha agonists (predominantly palmitoylethanolamide; PEA) significantly attenuated pain-associated behaviours in a broad range of inflammatory and neuropathic pain models. Fatty acid amide hydrolase (FAAH) inhibitors, monoacylglycerol lipase (MGL) inhibitors and cannabidiol (CBD) significantly attenuated pain-associated behaviours in neuropathic pain models but yielded mixed results in inflammatory pain models. The reporting of criteria to reduce the risk of bias was low, therefore the studies have an unclear risk of bias. The value of future studies could be enhanced by improving the reporting of methodological criteria, the clinical relevance of the models and behavioural assessments. Notwithstanding, the evidence supports the hypothesis of cannabinoid-induced analgesia.

Recent research on human placebo analgesia has suggested the need for rodent models to further elucidate the neural substrates of the placebo effect. This series of 3 experiments therefore was performed in an attempt to develop a model of placebo analgesia in rats. In each study, female Sprague-Dawley rats received an L5 spinal nerve ligation to induce a neuropathic pain condition. Each rat then underwent a 4-day conditioning procedure in which an active analgesic drug or its vehicle (unconditioned stimulus) was associated with the following cues (conditioned stimuli): novel testing room (environmental), vanilla scent cue (olfactory), dim incandescent lighting (visual), restraint procedure/injection (tactile), and time of day and injection-test latency (temporal). The analgesics for each experiment were as follows: Experiment 1 used 90 mg/kg gabapentin, experiment 2 used 3mg/kg loperamide hydrochloride, and experiment 3 used 6 mg/kg morphine sulfate. On the following test day, half of the animals received the opposite treatment, resulting in 4 conditioning manipulations: drug/drug, drug/vehicle, vehicle/drug, and vehicle/vehicle. Nociceptive thresholds were assessed with the mechanical paw withdrawal threshold test each day after the conditioning procedure. In all 3 experiments, no significant differences were detected on test day between control and placebo groups, indicating a lack of a conditioned placebo analgesic response. Our results contrast with prior research that implies the existence of a reliable and robust response to placebo treatment. We conclude that placebo analgesia in rats is not particularly robust and that it is difficult to achieve using conventional procedures and proper experimental design.

Introduction: Self-dosing of off-the-shelf cannabidiol (CBD) for a myriad of health conditions is common in the USA. These CBD products are often mislabeled, suggesting that much less or much more CBD is being consumed than indicated on the label. This study examined the relationship between long-term self-dosing of CBD and (a) indications and, when a verified concentration of CBD is being consumed, (b) the daily CBD dosage, (c) the impact on general health and symptoms, and (d) over-the-counter (OTC) and prescription (Rx) drug usage. Methods: US adults 18–75 years of age who had used unverified CBD products for >1 month were recruited to participate in this decentralized, observational, IRB-approved study and provided a concentration-verified CBD product of their choice from 15 different vendors for 4 weeks. Prior to receiving product, they were queried on their primary reason for use (PRfU), primary symptom for use (PSfU), general health score (GHS), symptom score (SS), OTC and Rx drug use, and daily CBD dose. Individuals were queried daily on OTC and Rx drug use and CBD dose and weekly on SS and GHS prior to (pre-CBD) and after (post-CBD) ingestion of CBD on that day. Results: The PRfU included chronic pain, mental health, general health and wellness, sleep disorders, the central nervous system, digestive health, and others, while the PSfU included anxiety, back and/or joint pain, sleep, inflammation, and others. The mean daily dose was normally distributed, with a mean, median, and range of 53.1, 40.8, 8–390 mg/day, respectively. For both GHS and SS, the post-CBD was significantly higher than the pre-CBD score for each category of PRfU. The GHS scores did not change over the study, but pre- and post-CBD SS improved over time, with pre-improving more than post-CBD SS. The percentage of individuals decreasing or completely stopping OTC drugs or Rx drugs over the 4 weeks was 31.2% and 19.2%, respectively, with those taking CBD for chronic pain, decreasing drug use the most. OTC and Rx drug usage decreased when the CBD dose was changed and when GHS and SS improved. Conclusion: Pain, mental health (primarily anxiety/stress), and sleep are the most common reasons for CBD use. Self-administration of CBD reduced OTC and Rx drug usage at daily doses less than those reported in controlled studies. CBD self-administration significantly improves self-perception of general health and decreases symptom severity, and as these improve, fewer OTC and Rx drugs are used.