The latest study published in the journal of Neurology Review has shown that sensitivity of patients to pain could be increased by using regular doses of codeine. The researchers during this study compared the pain reliving and pain worsening effects of codeine and morphine.
Codeine is continuously being used to reduce pain since 100 years but its safety and effectiveness has not yet been tested in this way said 
by University’s Professor Paul Rolan, who is also a headache specialist at the Royal Adelaide Hospital.
“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.”
“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.”
“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.”
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Source: University of Adelaide
Suggested Readings:
Hoeben E, Smit JW, Upmalis D, et al. Dose-response relationship after single oral dose administrations of morphine and oxycodone using laser-evoked potentials on UVB- and capsaicin-irritated skin in healthy male subjects. Pain. 2012;153(8):1648-1656.
Little JW, Chen Z, Doyle T, et al. Supraspinal peroxynitrite modulates pain signaling by suppressing the endogenous opioid pathway. J Neurosci. 2012;32(32):10797-10808.
Wala EP, Holtman JR, Sloan PA. Ultralow dose fentanyl prevents development of chronic neuropathic pain in rats. J Opioid Manag. 2013;9(2):85-96.
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