Paracetamol used to treat acute lower back pain is no better than a dummy pill, research in the Lancet suggests.
The largest trial to date suggests the drug does not improve recovery time or provide greater pain relief than a placebo.
The study questions whether paracetamol should remain a first-choice drug on most national guidelines.
But experts caution anyone considering a change of medication should seek medical advice.
About 26 million people suffer from lower back pain each year in the UK, and it remains a leading cause of disability worldwide.
Researchers studied more than 1,650 people across several primary care centres in Australia, who had experienced back pain for six weeks or less.
- A third received regular doses of paracetamol
- A third took the drug as needed
- A third were given a dummy pill (placebo) for one month
Paracetamol did not reduce the intensity of the pain, nor did it improve sleep quality. And scientists found there was no difference in recovery time for all three groups - an average of 17 days.
Lead author Dr Christopher Williams, from the University of Sydney, said: "The results suggest we need to reconsider the universal recommendation to provide paracetamol as a first-line treatment."
The researchers say the mechanisms behind lower back pain may differ to those in pain felt in other conditions such as headaches, toothaches and post-surgery discomfort - where there is evidence that paracetamol can provide relief.
And as people in the trial recovered more quickly than those in some previous studies, they suggest the advice and reassurance provided during their trial may be more effective than medication.
Dr Andrew Moore, a senior researcher at the Churchill Hospital, Oxford, who was not involved in the research, told the BBC: "Paracetamol does not work for every type of pain, nor does it work for every person.
"There has been evidence emerging for some time that paracetamol does not benefit most people with chronic back pain for example.
"And I'm willing to bet in 10 years' time national guidelines will have changed."
However, the researchers were unsure what to recommend instead due the side-effects of some anti-inflammatory pain killers and say people should discuss their options with their doctor.
Prof Roger Knaggs of the British Pain Society, told the BBC: "Despite how common paracetamol use is, this is a question that hasn't been studied in such a rigorous way before.
"For people who feel no benefit, there are alternative strategies. They should speak to their pharmacists or doctors and discuss any side-effects of other medication."
Prof Christine Lin, an author on the study, said: "Other ways to ease back pain include remaining as active as possible and avoiding bed rest."