Seven weeks of treatment with delta-9-atetrahydrocannabinol (THC) did not improve chronic abdominal pain in a placebo-controlled trial of 65 adults.

Treatment “was safe and well tolerated,” but did not significantly reduce pain scores or secondary efficacy outcomes, Marjan de Vries, MSc, and her associates wrote in the July issue of Clinical Gastroenterology and Hepatology ( doi: 10.1016/j.cgh.2016.09.147 ). Studies have not clearly shown that THC improves central pain sensitization, a key mechanism in chronic abdominal pain, they noted. Future studies of THC and central sensitization include quantitative sensory testing or electroencephalography, they added.

Treatment-refractory chronic abdominal pain is common after abdominal surgery or in chronic pancreatitis, wrote Ms. de Vries of Radboud University Medical Center, Nijmegen, the Netherlands. Affected patients tend to develop central sensitization, or hyper-responsive nociceptive central nervous system pathways. When this happens, pain no longer couples reliably with peripheral stimuli, and therapy targeting central nociceptive pathways is indicated. The main psychoactive compound of Cannabis sativa is THC, which interacts with CB1 receptors in the central nervous system, including in areas of the brain that help regulate emotions, such as the amygdala. Emotion-processing circuits are often overactive in chronic pain, and disrupting them might help modify pain perception, the investigators hypothesized. Therefore, they randomly assigned 65 adults with at least 3 months of abdominal pain related to chronic pancreatitis or abdominal surgery to receive oral placebo or THC tablets three times daily for 50-52 days. The 31 patients in the THC group received step-up dosing (3 mg per dose for 5 days, followed by 5 mg per dose for 5 days) followed by stable dosing at 8 mg. Both groups continued other prescribed analgesics as usual, including oxycontin, fentanyl, morphine, codeine, tramadol, paracetamol, anti-epileptics, and nonsteroidal anti-inflammatories. All but two study participants were white, 25 were male, and 24 were female.

At baseline, all patients reported pain of at least 3 on an 11-point visual analogue scale (VAS). By days 50-52, average VAS scores decreased by 1.6 points (40%) in the THC group and by 1.9 points (37%) in the placebo group (P = .9). Although a strong placebo effect is common in studies of visceral pain, that did not prevent pregabalin from significantly outperforming placebo in another similarly designed randomized clinical trial of patients from this study group with chronic pancreatitis, the investigators noted.

The THC and placebo groups also resembled each other on various secondary outcome measures, including patient global impression of change, pain catastrophizing, pain-related anxiety, measures of depression and generalized anxiety, and subjective impressions of alertness, mood, feeling “high,” drowsiness, and difficulties in controlling thoughts. The only exception was that the THC group showed a trend toward improvement on the Short Form 36, compared with the placebo group (P = .051).

Pharmacokinetic analysis showed good oral absorption of THC. Dizziness, somnolence, and headache were common in both groups, but were more frequent with THC than placebo, as was nausea, dry mouth, and visual impairment. There were no serious treatment-related adverse events, although seven patients stopped THC because they could not tolerate the maximum dose.

Some evidence suggests that the shift from acute to chronic pain entails a transition from nociceptive to cognitive, affective, and autonomic sensitization, the researchers noted. “Therefore, an agent targeting particular brain areas related to the cognitive emotional feature of chronic pain, such as THC, might be efficacious in our chronic pain population, but might be better measured by using affective outcomes of pain,” they concluded.

The trial was supported by a grant from the European Union, the European Fund for Regional Development, and the Province of Gelderland. The THC was provided by Echo Pharmaceuticals, Nijmegen, the Netherlands. The investigators reported having no conflicts of interest.