Paracetamol medications are no more effective than a placebo for the most common illnesses or injuries, a medical review of the drug has revealed.

The most ineffective use of paracetamol is when it is taken for back pain, University of Sydney researchers have concluded in a review for the Medical Journal of Australia.

Researchers conducted systematic reviews that compared evidence of the analgesic effects of paracetamol with placebo (saline solution or sterile water) in treating almost 50 common pain conditions.

It was discovered only knee and hip osteoarthritis, craniotomy, tension headache and perineal pain after childbirth were receptive to the drug.

"While paracetamol is widely used, its efficacy in relieving pain has been established for only a handful of conditions, and its benefits are often modest," the review found.

"High or moderate quality evidence that paracetamol [typically 0.5–1g, single or multiple doses] is superior to placebo for relieving pain was available for only four of 44 painful conditions examined."

Paracetamol is the most commonly used painkiller in Australasia but there is little to no evidence to support its ability to relieve pain associated with migraine, post-operative pain, dental procedures, childhood middle ear infections, back pain, abdominal pain and common cold-related headaches.

"For tension headaches we know it works better than placebo, but for most other conditions we simply lack the evidence to be able to make strong or definitive statements about paracetamol's effectiveness," said the review's lead author, Dr Christina Abdel Shaheed.

The MJA review is a blow to paracetamol, which in 2018 became the most dominant over-the-counter analgesic following a decision to make codeine medication prescription only.

"This MJA review is that for the first time it brings all the evidence on efficacy of paracetamol to treat pain together in one document," Shaheed said.

Marketing of paracetamol medications also needed to change, the review recommended.

"Back pain guidelines should stop recommending paracetamol," Shaheed said. "About 50 per cent of back pain guidelines still recommend paracetamol, even though we now know it is ineffective for back pain.

"Our review highlights the need for large, high-quality trials to reduce uncertainty about the efficacy of paracetamol for relieving common pain conditions."