June 24 (HealthDay News) -- Oral cannabis (a form of medical marijuana) was ineffective in treating certain types of acute pain and actually increased sensitivity to some other kinds of discomfort, say researchers at the Medical University of Vienna, Austria.

Their study included 18 healthy women who were given oral cannabis or a placebo. The women were then evaluated for heat and electrical pain thresholds in skin areas that had induced sunburn. This is an accepted method of assessing response to acute pain.

"The surprising result of our study was the absence of any kind of analgesic activity of THC-standardized cannabis extract on experimentally induced pain using well-established human model procedures," study author Dr. Birgit Kraft said in a prepared statement. "Our results also seem to support the impression that high doses of cannabinoids may even cause increased sensitivity in certain pain conditions."

The study is published in the July issue of the journal Anesthesiology.

Previous research has suggested that cannabis and tetrahydrocannabinol (THC -- the main psychoactive component of marijuana) may help ease chronic pain in cancer patients, spinal cord injury patients, and people with multiple sclerosis. There have been inconsistent findings about the effects on acute pain.

This new study's findings about oral cannabis and acute pain are seemingly conclusive, according to the researchers.

"From comparisons with previous clinical data, the lack of pain relief from the cannabis dosage and oral administration in our study cannot be considered the result of inadequate dosage or insufficient intestinal absorption," Kraft said. "The high levels of THC detected in the blood of our subjects as well as the occurrence of typical THC side effects argue for sufficient availability, and thus we draw the conclusion that THC was not effective in treating acute pain."

However, cannabis may remain a viable treatment option for certain types of chronic pain.

"Pain is a very complex and subjective phenomena," Kraft said. "Chronic pain has not only been shown to lead to changes in peripheral and central neural processing, but also to be associated with psychosocial problems, physical disorders, and functional disabilities. Recent studies have indicated that cannabis can be effective in treating certain types of chronic pain and helping patients to cope by improving quality of life."

SOURCE: American Society of Anesthesiologists, news release, June 24, 2008