by David Douglas

Last Updated: 2007-11-22 8:30:08 -0400 (Reuters Health)

NEW YORK (Reuters Health) – Moderate doses of smoked cannabis appear to be effective in reducing experimental pain in volunteers, according to researchers at the University of California, San Diego.

"Previous studies have suggested that smoked cannabis increases pain," lead investigator Dr. Mark Wallace told Reuters Health. "This is the first study using a dose-response method that suggests smoked cannabis has a therapeutic window with moderate doses decreasing pain and high doses increasing pain."

In the November issue of Anesthesiology, Dr. Wallace and colleagues describe the pain model they employed: intradermal capsaicin that results in the transient selective activation of C fibers. The injection causes a brief pain state that is replaced by an enlarged area of tactile allodynia and thermal hyperalgesia that persists for an extended interval.

In a double-blind randomized placebo-controlled crossover study, the researchers studied the effect of smoked cannabis (2%, 4%, and 8% 9-delta-tetrahydrocannibinol by weight) or placebo on pain and cutaneous hyperalgesia induced by intradermal capsaicin in 15 healthy volunteers. Capsaicin was injected into opposite forearms 5 and 45 minutes after drug exposure.

No dose of cannabis had any effect at 5 minutes, but by 45 minutes after exposure there was a significant decrease in pain with the medium (4%) cannabis dose and a significant increase with the high (8%) dose. No such effect was seen with the 2% dose, and none of the doses had any effect on the area of hyperalgesia.

Nevertheless, the researchers point out that no conclusions "on the analgesic efficacy of smoked cannabis on clinical pain states can be made from this study as the relationship between analgesic effects in experimental pain and clinical pain states is unknown."

Anesthesiology 2007.