Smoked cannabis was well tolerated and effectively relieved chronic neuropathic pain from HIV-associated sensory neuropathy. The findings are comparable to oral drugs used for chronic neuropathic pain.

The most obvious impairments associated with spinal cord injury (SCI) are loss of sensation and motor control. However, many subjects with SCI also develop persistent neuropathic pain below the injury which is often severe, debilitating and refractory to treatment. The underlying mechanisms of persistent neuropathic SCI pain remain poorly understood. Reports in amputees describing phantom limb pain demonstrate a positive correlation between pain intensity and the amount of primary somatosensory cortex (S1) reorganization. Of note, this S1 reorganization has also been shown to reverse with pain reduction. It is unknown whether a similar association between S1 reorganization and pain intensity exists in subjects with SCI. The aim of this investigation was to determine whether the degree of S1 reorganization following SCI correlated with on-going neuropathic pain intensity. In 20 complete SCI subjects (10 with neuropathic pain, 10 without neuropathic pain) and 21 control subjects without SCI, the somatosensory cortex was mapped using functional magnetic resonance imaging during light brushing of the right little finger, thumb and lip. S1 reorganization was demonstrated in SCI subjects with the little finger activation point moving medially towards the S1 region that would normally innervate the legs. The amount of S1 reorganization in subjects with SCI significantly correlated with on-going pain intensity levels. This study provides evidence of a link between the degree of cortical reorganization and the intensity of persistent neuropathic pain following SCI. Strategies aimed at reversing somatosensory cortical reorganization may have therapeutic potential in central neuropathic pain.

Tolerance to the anti-nociceptive effects of opioids develops rapidly in animals. In contrast, humans with chronic pain show little or no loss of pain relief in prospective opioid trials of 4-8 weeks duration. Employing the Brief Thermal Sensitization model to induce transient cutaneous secondary hyperalgesia, we tested the hypothesis that opioid analgesic tolerance would develop rapidly. In this outpatient randomized placebo-controlled study, subjects in the MMMMP group received two injections of subcutaneous morphine 6 mg (150 min apart) on Monday-Thursday (total 48 mg over 4 days) and matching saline placebo on Friday. Subjects in the PPPPM group received placebo on Monday-Thursday and morphine (total 12 mg) on Friday. Sixty-one healthy volunteers were enrolled; morphine side effects accounted for all nine non-completions. Compared to the first placebo day, the reduction in the area of secondary hyperalgesia on the first morphine day was significant and robust in both groups. Morphine suppression of the painfulness of skin heating and elevation of the heat pain detection threshold were also significant. During 4 days of twice-daily injections, the decline in anti-hyperalgesic effects of morphine did not reach statistical significance (p=0.06) compared to placebo. Morphine side effects did not correlate with anti-hyperalgesic effects and withdrawal symptoms did not emerge. As 4 days is the threshold for demonstrating analgesic tolerance to twice-daily morphine in animal models, a longer period of opioid exposure in healthy volunteers might be needed to detect analgesic tolerance.