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Last updated January 1, 2014

VIENNA (Reuters Health) - Men and women differ in how they respond to pain, and this may mean that painkiller regimens should be varied according to patient gender, researchers reported this week at the 9th World Congress on Pain.

An example of these differences was the discovery by Dr. Christine A. Miaskowski of the University of California, San Francisco, that a drug used to relieve pain after dental surgery can help women but actually increases pain in men.

The dental pain study compared the effects of 5 mg, 10 mg, and 20 mg doses of nalbuphine for pain relief following wisdom tooth extractions. Miaskowski reported that 5 mg of nalbuphine had no impact on women but had a "significant anti-analgesic effect in men." The 10-mg dose provided moderate pain relief in women and some pain relief in men, but pain relief from the drug ended significantly sooner in men than in women. The 20-mg dose had no effect on either gender.

Nalbuphine is relatively selective for the kappa type of opioid receptor in the brain, and Miaskowski said that her data support previous suggestions that "kappa-selective opioids appear to show a gender preference for females"

"This may lead us to begin to develop pain regimens that are specific by gender," she concluded.

In related work presented at the meeting Thursday, Dr. John Carmody said that gender also affects response to nonsteroidal anti-inflammatory drugs (NSAIDs). In their study, Carmody and colleagues at the University of New South Wales, Sydney, Australia, studied the effects of 400 mg and 800 mg doses of the NSAID ibuprofen on volunteers' tolerance thresholds for electrically-induced pain in the earlobe.

Young females were significantly more likely than young males to have no analgesic effect from the NSAID treatment, regardless of dose, Carmody announced. "This appears to validate our hypothesis that gender is an issue in interindividual response variability to NSAIDs," he said.

The researchers also reported that in female volunteers, "striking differences in analgesic responsiveness were observed at different phases of the menstrual cycle."

Also at the meeting, Dr. Jeffrey S. Mogil, of the University of Illinois, Champaign, reported studies of stress-induced analgesia that indicate that there is a female-specific mechanism of analgesia that is turned on or off by the presence or absence of estrogen, and, which appears to affect several types of pain receptors.

Mogil has identified a region on chromosome 8 that is female-specific and that accounts for over 50% of the variance in females, and is unimportant in males. His group has identified a candidate gene in this area and is working on confirming that this is the gene that can be switched on or off by estrogen and that governs this pain response. "There may be fundamental differences in the ways males and females modulate pain," Mogil said.

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