Researchers found frequent users of codeine were complaining that they were getting poor pain relief despite using bigger doses. Rather than improving the pain, it was actually getting worse.
Codeine, the most widely used strong pain relief medication in the world, is available on prescription and over the counter in many products, including Nurofen Plus and Solpadeine.
Around 27million pills containing codeine are sold in Britain every year in a market worth £500million.
Official figures show more than 30,000 people in the UK have become addicted to drugs containing codeine, which led to warnings being put on packets in 2009.
Packet sizes for over-the-counter products were also limited that year to just 32 tablets, with larger packs available only by prescription.
Consumers are told not to take them for more than three days at a time because of fears of misuse.
But now new research suggests the drug is less effective than previously thought.
A study comparing the effect of codeine and morphine, both opioid drugs which target the central nervous system, found that codeine provides much less pain relief, but resulted in the same level of increased sensitivity to pain.
Headache specialist Professor Paul Rolan, of the Royal Adelaide Hospital in Australia, said that codeine has been widely used as pain relief for more than 100 years but its effectiveness has not been laboratory tested in this way before.
He said: ‘In the clinical setting, patients have complained their headaches became worse after using regular codeine, not better.
‘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 that it was about time we looked into how effective it really is.
Jacinta Johnson, a researcher at the university, said a major issue for users of opioid drugs is the more you take, the more it can increase your sensitivity to pain, so you may never get the level of relief you need.
She added: ‘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.
‘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.’
While she admitted that more research is needed, the findings suggest a potential problem for anyone suffering from chronic pain such as arthritis who needs regular treatment with painkillers.
Professor Rolan added: ‘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.
‘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 research was presented at the 2013 International Headache Congress in Boston.
A clinical trial testing a new approach to treating codeine-related headaches is now being run by Professor Rolan.
Higher-dose codeine has to be prescribed by a doctor. Products containing co-codamol – paracetamol and low-dose codeine – are available to buy over the counter.
Earlier this year doctors were warned not to prescribe codeine for children under 12 because of the risk of potentially deadly side effects, including breathing problems.
Over-the-counter medicines containing codeine also warn they should not be used by children.
The comedian Mel Smith, who died of a heart attack in July aged 60, said he was addicted to the codeine in Nurofen Plus, which he called his ‘dark secret’