Sunday 7 September 2014

Paracetamol (Acetaminophen) Does Not Work For Low Back Pain?

This is the message I heard being touted recently by a study author while listening to the Australian ABC news radio. Could this be? Have we been prescribing this medication inappropriately for all these years?

As it turns out, the devil is in the details. It all depends upon which question you are asking.

Enter a large multicentre RCT across 235 primary care centres in Sydney, Australia. 1652 patients with new low back pain were randomized in a 1:1:1 ratio to receive up to 4 weeks of regular paracetamol, as-needed paracetamol or placebo. Patients with suspected serious spinal pathology were excluded from the study. The methods were quite good and the double-dummy blinding was quite clever.

Patients were followed up at 1 week and participants recorded pain scores in to a daily pain and drug diary until recovery or 4 weeks.

The primary outcome was time to recovery in days. Recovery was defined as the first day of 0 or 1 pain intensity, measured on a 0-10 point scale, maintained for 7 consecutive days.

Did I read this correctly? The study authors were trying to determine if paracetamol could cure low back pain? I had no idea that there were those who thought paracetamol could cure low back pain. I was always under the impression that it might take the edge off of the pain a little bit for a brief period of time. But a cure, no.

The authors justified this primary outcome as below.

We chose this outcome on the basis of previous findings of fast recovery in a cohort receiving regular paracetamol. As such, we postulated that regular paracetamol would improve recovery by decreasing pain intensity and allowing people with acute low back pain to remain active and resume normal movement as soon as possible.

The results showed a median time to recovery of 17 days with no statistical difference between the groups. So, paracetamol does not cure low back pain. Big surprise.

To be fair, the study did look at secondary outcomes of pain intensity, disability, function, global rating of symptoms change, sleep quality and quality of life and found no differences. But one could reasonably question the validity of the measuring tools. In addition, one should always be a bit more skeptical when looking at multiple secondary outcomes.

One should also remember that "placebo" is not an inactive treatment arm. To put another way, placebo is not nothing. It can have substantial physiologic and psychological effects in some people especially when it comes to pain. This may have watered down the results of the primary outcome. It would have been interesting if the study had a "nothing" arm. 

In the end, this study truly emphasizes the need for clinicians to be vigilant and emphasizes the value of taking a critical look at the medical literature. If one was not aware of the study’s primary outcome, one might be convinced there is no role for paracetamol for low back pain. This is far from the truth. Nevertheless, I don’t think anyone is deluded that paracetamol does very much. But at least it is cheap, has a low side effect profile and might help for a little bit for a brief period of time.

What’s the take home message? Paracetamol is still fine for low back pain and beware what you hear on the radio.


Williams CM, Maher CG, Latimer J, et al. Efficacy of paracetamol for acute low back pain: a double-blind, randomised controlled trial. Lancet 24, 2014;  [Epub ahead of print]


Williams CM, Latimer J, Maher CG, et al. PACE--the first placebo controlled trial of paracetamol for acute low back pain: design of a randomised controlled trial. BMC Musculoskelet Disord 2010;11:169

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