Why do opiates affect the human neurosystem is such a way as to relieve pain and cause euphoria?

I also read a story of a medical study in great Britian that researched long term use of opiates and found that in moderate doses and administered hygenically the users had no more health problems than the general population of non users. Any Comments?

: http://alcoholdrugstreatment.info/why-do-opiates-affect-the-human-neurosystem-is-such-a-way-as-to-relieve-pain-and-cause-euphoria