A "pandemic of over-regulation" of opioid-based painkillers such as morphine and fentanyl means billions of cancer patients around the world suffer intolerable pain, according to an international study.
Describing what they said was a "scandal of global proportions", researchers from the Global Opioid Policy Initiative say that governments that over-regulate such painkillers should consider the unintended consequences of restricting access to the medicines and change their approach.
According to a global analysis published in the Annals of Oncology journal, more than four billion people live in countries - many of them in emerging and developing regions - where regulations covering opioid-based painkillers leave patients in excruciating pain. The regulations often are imposed to staunch drug addiction.
"This is a tragedy born out of good intentions," said the study's lead author, Nathan Cherny, from Israel's Shaare Zedek Medical Centre in Jerusalem.
"When opioids are over-regulated, the precautionary measures to prevent abuse and diversion are excessive and impair the ability of health care systems to relieve real suffering," Cherny said.
The study, conducted in Africa, Asia, the Middle East, Latin America and the Caribbean, studied the availability of the seven opioid medications considered by the World Health Organisation to be essential for the relief of cancer pain.
The drugs studied were codeine, oral oxycodone, transdermal fentanyl, immediate and slow-release oral morphine, injectable morphine and oral methadone.
As well as being very effective painkillers, opioids are highly addictive. One of the best-known drugs is Oxycontin, a long-acting form of oxycodone which has prompted widespread drug addiction in the United States.
The study found that while there were problems with the supply of opioids in many countries, the main problem was over-regulation, making it difficult for doctors to prescribe and administer the medicines for legitimate use.
The "study has uncovered a pandemic of over-regulation in much of the developing world that is making it catastrophically difficult to provide basic medication to relieve strong cancer pain", Cherny says.
"Most of the world's population lacks the necessary access to opioids for cancer pain management and palliative care, as well as acute, post-operative, obstetric and chronic pain."
US drug regulators said in September that labels on long-acting opioids should contain stronger warnings highlighting the dangers of addiction and abuse.
James Cleary, of the Palliative Medicine at the University of Wisconsin Carbone Cancer Centre, who also led the study, says regulatory reform should be coupled with education of health workers about safe and responsible use of opioid drugs.
"The next step is for international and local organisations working alongside governments and regulators to thoughtfully address the problems," he said.
The authors pointed to Ukraine as one example of a country which had very restrictive laws, but is now allowing greater access to opioids.