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Low back pain is a common reason for missed work time. (Evgeny Atamanenko/Shutterstock)

​​The commonly-prescribed drug acetaminophen or paracetamol does nothing to help low back pain, and may affect the liver when used regularly, a large new international study has confirmed.

Reporting in today's issue of the British Medical Journal researchers also say the benefits of the drug are unlikely to be worth the risks when it comes to treating osteoarthritis in the hip or knee.

"Paracetamol has been widely recommended as being a safe medication, but what we are saying now is that paracetamol doesn't bring any benefit for patients with back pain, and it brings only trivial benefits to those with osteoarthritis," Gustavo Machado of The George Institute for Global Health and the University of Sydney, tells the Australian Broadcasting Corporation.

"In addition to that it might bring harm to those patients."

Most international clinical guidelines recommend acetaminophen as the "first choice" of treatment for low back pain and osteoarthritis of the hip and knee.

However, despite a trial last year questioning the use of acetaminophen to treat low back pain, there has never been a systematic review of the evidence for this.

Machado and colleagues analyzed three clinical trials and confirmed that acetaminophen is no better than placebo at treating low back pain.

An analysis of 10 other clinical trials by the researchers quantified for the first time the effect acetaminophen has on reducing pain from osteoarthritis in the knee and hip.

"We concluded that it is too small to be clinically worthwhile," says Machado.

He says the effects of acetaminophen on the human body are not well understood and just because it can stop headaches, it doesn't mean the drug will work in all circumstances.

"There is probably a difference in the pain mechanisms in low back pain and osteoarthritis, compared to headache," says Machado.

Liver effects

Importantly, the new study was the first to show that patients using acetaminophen for low back pain and osteoarthritis were nearly four times more likely than those taking placebo to have abnormal results on liver function tests.

Machado says it's unclear whether this means acetaminophen could cause liver damage in the long term.

"But if you see elevation of enzymes in the short term, it's a concern for the long term," he says.

Machado and colleagues point to another recent study suggesting acetaminophen raises the risk of cardiovascular, gastrointestinal and renal disease. They argue doctors should reconsider their recommendation that patients use the drug for low back pain and osteoarthritis of the hip or knee.

Keep active advice

"It's time the clinical guidelines are reviewed," says co-author of the new study associate professor Manuela Ferreira. "Paracetamol shouldn't be included in the guidelines for back pain."

When it comes to treating osteoarthritis, Machado and Ferreira call on doctors to explain the actual risks and benefits.

"If you ask me it's not worthwhile," says Ferreira.

Ferreira says anti-inflammatories are the second choice of treatment for low back pain but they have greater side- effects.

The researchers say other non-drug treatments recommended in the clinical guidelines should be used instead.

These include reassuring patients that low back pain is generally benign and that the best remedy is to keep active. Exercise, strength training and weight management have been shown to be effective in treating osteoarthritis of the hip and knee," says Machado.

Arthritis Research UK also said physical activity is probably a better and more effective way of keeping the pain of arthritis and joint pain at bay than taking painkillers.  

The arthritis group said it's been known for some time acetaminophen may not work for everyone with severe pain from their arthritis, but some people find it allows them to sleep and to exercise without discomfort.

Low back pain and other musculoskeletal conditions account for one-third of missed work time in Canada.

With files from CBC News