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Potential for severe liver damage
"Our systematic review found that paracetamol is ineffective for patients with low back pain and only provides negligible benefits for patients with lower limb osteoarthritis," said lead study author Gustavo Machado, a researcher at the George Institute for Global Health in Sydney, Australia.
Millions of people worldwide take the drug – known as acetaminophen (Tylenol) in the U.S. and Japan and paracetamol (Panadol) elsewhere – to relieve fevers as well as common aches and pains, often purchasing it without a prescription or guidance from a clinician.
Because of side effects, including the potential for severe liver damage, many doctors recommend that patients limit use to avoid accidental overdoses.
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"Acetaminophen, used properly, is one of the safest medications available to treat pain, but used improperly and excessively it can certainly be dangerous and lead to serious and life-threatening complications," Dr. Michael Hodgman, a toxicologist at the Upstate New York Poison Centre who wasn't involved in the study, said in an email.
Machado and colleagues analysed data from 13 previously published trials exploring the safety and effectiveness of acetaminophen/paracetamol for spinal pain and osteoarthritis.
The researchers reviewed the effectiveness of the drug for osteoarthritis of the hip or knee in ten trials covering a total of 3,541 patients. They also looked at three trials of lower back pain with 1,825 participants, combined.
All but one of the studies involved tablets or capsules, while one reported results of intravenous paracetamol for participants with lower back pain.
In ten of the trials, the daily dosage was about 3,900 to 4,000 milligrams, while three studies looked at about 3,000 milligrams per day.
No better than a placebo
In a pooled analysis, the researchers found paracetamol no better than a placebo for lower back pain and only slightly better for osteoarthritis, an age-related breakdown of cartilage and bone at the joints around the hips and knees that causes pain and stiffness.
The researchers also looked at side effects across nine trials and found no difference between paracetamol and placebo.
Combined results from three trials, however, showed that patients taking paracetamol were four times more likely to have abnormal liver function test results than those on placebo.
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"There is some emerging evidence on harms, showing that paracetamol is associated with increased risk of premature death, and cardiovascular, gastrointestinal and kidney disease," Machado said in an email. Still, clinical guidelines around the world generally recommend the drug as good first choice for treating pain based on the belief that it is safe, effective and affordable, he said.
Machado's team acknowledge some limitations in their analysis, including the small number of trials included and their lack of long-term follow-up.
The study also focused on chronic pain, and doesn't provide any insight into how the drugs work for acute pain, Dr. Richard Dart, director of the Rocky Mountain Poison and Drug Centre, said in an email.
Dart, who wasn't involved in the study, also cautioned against reading too much into the liver findings in the analysis, noting that no one had liver failure.
Weak anti-inflammatory
In addition, patients need to understand that, unlike aspirin and non-steroidal anti-inflammatory drugs, paracetamol isn't designed to relieve inflammation. It's a "weak anti-inflammatory agent but efficacious fever medication, mainly for children," said Dr. Martin Wehling, managing director of the Institute of Experimental and Clinical Pharmacology and Toxicology at the University of Heidelberg in Germany.
"Back pain has little to do with inflammation, therefore paracetamol/acetaminophen may only work in those unidentified patients in whom chronic processes may cause osteoarthritis even in the back," said Wehling, who wasn't involved in the study, in an email.
It may make sense, experts said, for patients to try low doses of paracetamol to relieve pain and to continue taking it if it works, as long as they don't exceed the recommended daily dose.
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"It is still a valuable medication as its toxicity is almost entirely restricted to overdose issues," Wehling said.
"If we had less dangerous medications we would certainly remove paracetamol/acetaminophen from the guidelines, but we do not have safer alternatives."
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