Frequent and large does of codeine may increase sensitivity to pain and fail to offer the same relief as morphine, according to a study presented at the 2013 International Headache Congress in the US.
Researchers from the Discipline of Pharmacology at The University of Adelaide in Australia conducted what they say is the world's first experimental study to compare both codeine and morphine in order to determine their pain-relieving and pain-increasing effects.
Codeine is a pain medication that is a part of the class of drugs known as opioids. It is commonly used to treat mild to moderately severe pain, particularly headaches.
But according to the researchers, patients have been known to complain that their headaches have worsened following regular codeine usage.
"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," says Professor Paul Rolan of the University of Adelaide and headache specialist at the Royal Adelaide Hospital.
Codeine shows same pain sensitivity as morphine
For the study, the researchers conducted a series of mouse experiments to see whether codeine induces increased pain sensitivity (hyperalgesia) and allodynia - pain caused by a non-injurious stimulus - in comparison to morphine.
Experiments involved testing various painkiller combinations on groups of male mice who were randomized to undergo morning and evening treatments over a series of 4 days.
The researchers conducted von Frey testing (fine-gauge metal wires to test pain sensitivity) on the mice and hot plate tests. A group of mice who underwent chronic constriction injury surgery were also monitored for their responses to codeine and morphine.
Results of the experiments revealed that codeine provided significantly less pain relief compared with morphine, but it showed the same level of increased pain sensitivity.
Jacinta Johnson, of the University of Adelaide and study author, explains:
"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."
She adds that although codeine and morphine are both opioids, codeine is a "trojan horse" drug, as 10% of it is converted to morphine, resulting in pain relief.
"However," she continues, "despite not offering the same level of pain relief, we found that codeine increased pain sensitivity just as much as morphine."
Potential issue for regular users
Prof. Paul Rolan notes that although further research is needed to better understand the effects of codeine, the findings of this study present a potential problem for those who require ongoing pain 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," he 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."
Medical News Today recently reported that the US Food and Drug Administration (FDA) has announced it will make changes to safety labeling and post-market study requirements for extended-release and long-acting opioids.