Opioids may be no more beneficial for treating cancer-related pain than aspirin or even a placebo, a study has found.

The trial led by researchers from the University of Sydney is one of few in the world to compare opioids such as morphine, oxycodone and methadone with a placebo.

Published on Tuesday, the study revealed found major gaps in evidence supporting the use of opioids to treat pain associated with cancer and recommended doctors take a more tailored approach when prescribing pain relief.

Opioids commonly thought of as weaker, such as codeine, or nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, piroxicam, ketorolac, diclofenac and the antidepressant medicine imipramine may be just as good as "powerful" opioids for treating background cancer pain, the study found.

Prescribing opioids comes with the risk of dependence and other negative side affects, making it crucial doctors prescribe them only when necessary.

Despite this, international bodies including the World Health Organisation recommend opioid medications to manage pain caused by cancer.

The team behind the study says the findings don't mean doing away with opioids, rather reconsidering a "one-size-fits-all" approach.

"The hope is that the findings can help guide doctors and patients to choose between different opioid treatment for cancer pain and empower individuals to consider alternatives if they are unable to tolerate opioid medicines or choose not to take them," said study senior author Mark Sidhom, from the Cancer Therapy Centre at Sydney's Liverpool Hospital.

The study found with moderate certainty the opioid tapentadol works better than a placebo for background pain caused by cancer.

It also found fentanyl used as a nasal spray or or oral spray may be more effective than placebo - although not for regular use.

"In practise, opioids are indispensable for intractable pain and distress at the end of life," Professor Jane Ballantyne, from the University of Washington in the US, said.

"What is worth highlighting is that non-opioids, particularly NSAIDs, are surprisingly effective for some cancer pain."

The study examined data from more than 150 published clinical trials and found only a small number used placebos.

Lead researcher Christina Abdel Shaheed said despite ethical challenges, placebo trials were necessary to guide clinical decision making.