Codeine can increase users' sensitivity to pain

Using large and frequent doses of the common pain-killer codeine may actually produce heightened sensitivity to pain, without the same level of relief offered by morphine, according to a new research.
Researchers from the University of Adelaide have conducted what is believed to be the world's first experimental study comparing the pain relieving and pain worsening effects of both codeine and morphine.
The University's Professor Paul Rolan, who is also a headache specialist at the Royal Adelaide Hospital, said codeine has been widely used as pain relief for more than 100 years but its effectiveness has not been tested in this way before.

"In the clinical setting, patients have complained that their headaches became worse after using regular codeine, not better," said Rolan.
"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," Rolan said.
In laboratory studies, University of Adelaide PhD student Jacinta Johnson found that 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," Johnson said.

"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," Johnson said.
"Both codeine and morphine are opioids but codeine is a kind of 'Trojan horse' drug - 10 per cent 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," Johnson said.
Rolan said while more research is needed, these laboratory findings suggest a potential problem for anyone suffering from chronic pain who 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," Rolan said.
"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," Rolan said.