Pain Sensitivity Heightened By Continued Use Of Codeine
September 13, 2013

Codeine Use Over Time Can Increase Sensitivity To Pain

April Flowers for - Your Universe Online

Researchers at the University of Adelaide's Discipline of Pharmacology have found that a heightened sensitivity to pain might result from using large and frequent doses of the pain-killer codeine, without the same relief offered by morphine.

The findings, published in Neurology Reviews, are the result of what is believed to be the world's first experimental study comparing the pain relieving and pain worsening effects of both codeine and morphine.

Although, codeine has been widely used as pain relief for more than a century, its effectiveness has not been tested in this manner before.

"In the clinical setting, patients have complained that their headaches became worse after using regular codeine, not better," Professor Paul Rolan, who is also a headache specialist at the Royal Adelaide Hospital, says.

"Codeine use is not controlled in the same way as morphine, and as it is the most widely used strong pain reliever medication in the world, we thought it was about time we looked into how effective it really is."

Jacinta Johnson, University of Adelaide PhD student, conducted laboratory studies that showed codeine provided much less pain relief than morphine, but resulted in the same level of increased sensitivity to pain.

"Pain sensitivity is a major issue for users of opioid drugs because the more you take, the more the drug can increase your sensitivity to pain, so you may never quite get the level of relief you need. In the long term it has the effect of worsening the problem rather than making it better. We think that this is a particular problem in headache patients, who seem more sensitive to this effect," Ms Johnson says.

"Both codeine and morphine are opioids but codeine is a kind of 'Trojan horse' drug – 10% of it is converted to morphine, which is how it helps to provide pain relief. However, despite not offering the same level of pain relief, we found that codeine increased pain sensitivity just as much as morphine."

More research is needed, according to Rolan, however the findings from this study suggest a potential challenge for anyone suffering from chronic pain that needs ongoing medication.

"People who take codeine every now and then should have nothing to worry about, but heavy and ongoing codeine use could be detrimental for those patients who have chronic pain and headache," Professor Rolan says. "This can be a very difficult issue for many people experiencing pain, and it creates difficulties for clinicians who are trying to find strategies to improve people's pain."

The results of this study were presented at the 2013 International Headache Congress in Boston by Ms. Johnson. Professor Rolan is continuing the research with a clinical trial to test a new approach to treating codeine-related headache.