New Adelaide Uni research has found large, frequent codeine doses can increase pain sensitivity LARGE and frequent doses of the painkiller codeine can increase pain sensitivity, without giving as much relief as morphine, new University of Adelaide research has found. The university believes it has conducted the world's first experimental study comparing the pain relieving or worsening of the two drugs. The university's Professor Paul Rolan, who is also a headache specialist at the Royal Adelaide Hospital, said codeine had been widely used as pain relief for more than 100 years but its effectiveness had 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," Professor Rolan 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. PhD student Jacinta Johnson conducted laboratory studies which found 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," she 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. Prof Rolan said the laboratory findings suggest a potential problem for anyone suffering from chronic pain who needs ongoing medication. He said more research was needed. "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. Ms Johnson presented this research at the 2013 International Headache Congress in the United States. Professor Ryan is also operating a clinical trial testing a new approach to treating codeine-related headache. For more information visit www.adelaide.edu.au/painresearch/participate .