A study of the effects of cannabis on regular smokers found that men experienced reduced sensitivity to pain after consuming the drug, but women did not. Although the study provides no explanation for the difference, it adds to evidence that there are differences between male and female pain pathways.
Forty two people who smoke marijuana regularly were given either ordinary marijuana or a placebo of zero-THC cannabis in a double blinded study and then asked to put their hand in very cold water. Participants reported when they first felt pain, to measure sensitivity, and were measured for the time they were able to keep their hand in the water, to measure tolerance, both commonly used pain tests. In addition, participants were asked to describe their experience of pain. The results were published in the journal Drug and Alcohol Dependence.
Although the women were slightly more able to tolerate the pain when after smoking the active marijuana, they reported no decrease in sensitivity to pain. Men, however, described experiencing less pain and were also able to hold on for longer when given the real thing, with clearly statistically significant differences in both cases.
"These findings come at a time when more people, including women, are turning to the use of medical cannabis for pain relief," said Dr Ziva Cooper of Columbia University in a statement. "Preclinical evidence has suggested that the experience of pain relief from cannabis-related products may vary between sexes, but no studies have been done to see if this is true in humans."
Dr Cooper's research focus is on the differences in male and female responses to recreational drugs, rather than in pain transmission. For this study she found no statistically significant differences in how stoned the men and women reported feeling, although this may have reflected the small sample size.
Last year researchers found that microglia cells, which are mostly known as the brain's immune system, transmit pain signals in male mice, but seem not to play the same role for females. The search for a matching female pathway is in its infancy.
If the same transmission difference applies in humans it is logical that certain substances will act as painkillers in one sex and not in the other. This research suggests that THC molecules are the first examples we have found to fit that category.
"This study underscores the importance of including both men and women in clinical trials aimed at understanding the potential therapeutic and negative effects of cannabis, particularly as more people use cannabinoid products for recreational or medical purposes," Cooper said. The observation may have broader application. Many clinical trials are only conducted on men in order to remove confounding effects from the menstrual cycle, but an increasing number of researchers are questioning the assumption that studies done in this way are applicable to women, and this may be particularly true for pain research.