Paracetamol does not stop lower back pain or help the ache of arthritis and doctors should consider prescribing exercise instead, a major review has found.
The over-the-counter painkiller, which is taken by millions of people in Britain for back problems, was found to be no better than placebo.
Around 26 million people suffer from back pain in Britain while nearly nine million have sought medical help for osteoarthritis, the most common form of arthritis.
But although clinical guidelines recommend paracetamol as the first line drug treatment for both conditions it has little effect and could cause long term liver damage, the University of Sydney warned in the British Medical Journal.
Lead author Gustavo Machado said the new research showed that guidance must be changed.
"Paracetamol is the most widely used over-the counter medicine for musculoskeletal conditions so it is critical that we review treatment recommendations in light of this new evidence," he said.
"In our research, paracetamol for low back pain and osteoarthritis was also shown to be associated with higher risk of liver toxicity in patients.
"Patients were nearly four times more likely to have abnormal results on liver function tests compared to those taking placebo pills."
In 2013 the National Institute for Health and Care Excellence (Nice) warned that continued use of paracetamol could lead to heart, kidney and intestinal problems, and recommended GPs only prescribe the lowest effective dose for the shortest possible time.
However the drugs regulator was forced into a u-turn last year following criticism from the Royal College of GPs and arthritis charities who said it would have a drastic impact on GP pain management.