Sex differences in response to opioid drugs

It appears that women respond better to morphine than men and men respond better to a different opioid (narcotic) drug, butorphanol.  This was the conculsion of a study presented at the American Pain Society.  The next step that the researchers plan is to look at possible genetic factors which may explain this difference.  Although the study was small and needs to be confirmed, such knowledge could have an important practical impact.  Opioid drugs are rarely used for the treatment of headaches, but when necessary it would be useful to know which one has a better chance of success.