By Anne Harding

NEW YORK (Reuters Health) - Giving carefully calibrated doses of smoked marijuana to people with neuropathic pain, which can be difficult-to-treat and extremely painful, can ease their pain without clouding their minds, California researchers report.

Neuropathic pain can result from spinal cord injury, diabetes-related nerve damage, multiple sclerosis, or other types of nerve injury, and is typically treated with a wide range of drugs including antidepressants, anticonvulsants, opioids, and anti-inflammatories, the study's lead author, Dr. Barth Wilsey of the University of California, Davis Medical Center, told Reuters Health. Wilsey became interested in testing marijuana for treating neuropathic pain, he said in an interview, after many of his patients told him they were already smoking pot to cope.

To examine the pain-fighting effects of pot scientifically, he and his colleagues had 38 people with neuropathic pain smoke high-dose joints containing 7% delta-9-tetrahydrocannabinol (9-THC), a lower dose version containing 3.5% 9-THC, or a placebo cigarette from which all 9-THC had been extracted.

Each study participant went through a trial of each of the three cigarettes, using a standardized system for puff timing and inhalation length developed at the University of California Center for Medicinal Cannabis Research. All study participants had abstained from smoking pot for at least 30 days beforehand, and smoking sessions were separated by at least 3 days to allow the drug to leave their systems.

The low- and high-dose marijuana cigarettes produced identical levels of pain relief and reductions in the intensity and unpleasantness of pain, the researchers found. But study participants were more likely to report feeling "high," "stoned" or "impaired" when smoking the higher-dose joint. They also had impairments in attention, learning, memory and fine-motor coordination and speed. The lower-dose cigarettes produced some impairment in learning and memory, but to a lesser degree.

"The lower dose did not adversely affect people's thinking," Wilsey said. "There might be a therapeutic window that we could advise for using smoked cannabis in treating nerve injury pain."

Nevertheless, he and his colleagues note, medical marijuana should only be prescribed with "caution" for treating neuropathic pain, "especially in instances in which learning and memory are integral to a patient's work and lifestyle." The drug's use in younger patients should also be examined carefully, they add, given that smoking pot has been shown to increase the risk of psychosis in some people.

Wilsey and his team are now planning studies of marijuana cigarettes containing 1.75% 9-THC to determine if an even lower dose can produce equal pain relief with fewer side effects. They also are planning to test vaporized cannabis, to avoid the health effects of smoke.

SOURCE: The Journal of Pain, April 17, 2008.

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