In an animal study, the scientists sought to confirm the age-related difference in tolerance to opioid drugs. They note that no study had systematically evaluated how quickly rats develop tolerance to opioid pain medication as a function of age. Rats aged three weeks, three months, six months and a year were given twice-daily injections of morphine proportional to their weight, and pain-relief was assessed in a standard procedure of applying heat to the rat's tail and measuring how quickly the animal flicked its tail away from the heat source.
The researchers found that the youngest rats experienced drug tolerance more than five times sooner than the oldest rats. Six-month-old and three-month-old rats showed a 250 percent and a 150 percent increase in time to tolerance compared with the three-week-old rats, they reported. The findings are consistent with the human studies in which older patients continue to experience benefit from a given opioid dosage while younger patients need to escalate the dosage.
Why the older patients found long-lasting pain relief at lower doses -- a finding confirmed in the animal studies -- is not certain, Palmer says. But the results suggest to the UCSF scientists that use of opioids for pain relief in both the young and the old should be reconsidered.
For older patients, opioids appear to be a safe, effective medication to treat long-lasting, serious pain without the increased risks posed by Cox-2 inhibitors and other anti-inflammatory drugs, Palmer concludes. On the other hand, younger people should be cautious of using daily opioids to treat chronic pain.
"Younger people are more vulnerable to opioid tolerance and ever-escalating doses, and they are at much lower risk than older patients for heart attack and gastrointestinal bleeding linked to anti-inflammatory drugs," she said. In sum, younger patients may be better candidates for the newer non-steroidal anti-inflammatories and poorer candidates
'"/>
Contact: Wallace Ravven
wravven@pubaff.ucsf.edu
415-476-2557
University of California - San Francisco
20-May-2005