Paracetamol for acute low-back pain

This article was published in Medical Observer.

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Clinical scenario

Karen, a 45-year-old registered nurse presented with acute low-back pain, a day after awkwardly transferring a patient.  She was otherwise well with no red flags.  In an online discussion forum (GPs Down Under), I recalled reading about a new systematic review in the BMJ that claimed that paracetamol was ineffective for low back pain. ^[1]  Is this true?

Clinical question

Is paracetamol effective at reducing pain and/or disability in acute low-back pain?

What does the existing research evidence say?

Step 1: The Cochrane Library

The Cochrane Library did not have a systematic review for paracetamol.  There was a review for NSAIDs that concluded – “moderate evidence that NSAIDs are not more effective than paracetamol for acute low-back pain, but paracetamol had fewer side-effects”. ^[2]

Step 2: The Trip Database

A PICO search using the TripDatabase (search term: P: “low back pain”, I: “paracetamol”, C: “placebo”, O: “pain”) found the PACE study – which was a randomised trial of paracetamol for acute low-back pain in Australian primary care. ^[3]

This was from the same research group who published the aforementioned systematic review in the BMJ. ^[1]  As over 90% of the participants in that part of the review was from this one trial, we should examine the PACE study by Williams et al. (2014) in detail. ^[3]

Critical appraisal

I will use the randomised controlled trial appraisal sheet from the Centre for Evidence Based Medicine. ^[4]

PICO

Participants: who was studied?

1643 adult patients (mean age: 45 years, 53% male), who presented to a primary care clinician (181 GPs, 50 pharmacists, 4 physiotherapists) in Sydney Australia, with acute low-back pain (pain less than 6 weeks duration, preceded by 1 month of no pain), recruited between Nov 2008 and Mar 2013.  Important exclusions: suspected serious spinal pathology, current use of full, regular recommended doses of an analgesic, spinal surgery in the preceding 6 months, and pregnancy.

Intervention: what was the exposure?

Regular paracetamol group:
2 × paracetamol 665 mg modified release tabs, 3 times/day, and
1-2 × placebo tabs every 4-6 hours (max: 8/day)

As-needed paracetamol group:
1-2 × paracetamol 500 mg tabs every 4-6 hours (max: 8/day), and
2 × placebo tablets, 3 times/day

Comparator: what was the control/alternative?

Double-placebo group:
2 × placebo tablets, 3 times/day, and
1-2 × placebo tabs every 4-6 hours (max: 8/day)

Outcomes: what was measured?

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

Secondary outcomes included: pain intensity, disability, drug adherence

Internal validity: are the trial results valid?

Randomised patient assignment?

Yes.  This appeared well performed and described (p. 1587). ^[3]

Groups similar at the start?

Yes.  The groups were very similar at baseline (Table 1, p. 1589). ^[3]

Groups treated equally apart from assigned treatment?

Yes.

All patients accounted for?

Yes.  Few participants dropped out of the study.

Measures objective?  Or patients and clinicians kept blinded?

Yes.  The participants appear to have been successfully blinded to their group allocation (Table 2, p. 1590). ^[3]

What were the results?

Primary outcome: there were no significant differences in the time to recover between the three groups (p = 0.79).  The median time to recovery was 17 days.

Secondary outcomes:

• there were no significant differences in pain intensity or disability between any of the groups at any of the measured time points
• there were no significant differences between the groups for adverse effects
• the median dose equivalent of paracetamol in week 1 was 3500 mg/day

Conclusion

This is an excellent Australian study with a challenging finding – regular paracetamol is likely ineffective for acute low-back pain.  The recently reported systematic review came to the same conclusion. ^[1]

The recommendation in current guidelines of using full adult doses of paracetamol regularly as first-line care for low-back pain ^[5] needs to be questioned.  There may be no good alternatives – NSAIDs have a small benefit over placebo, trends towards superiority over paracetamol, but unambiguously have more side-effects. ^[2]

After examining the evidence, I no longer assertively recommend full regular doses of paracetamol.  I place more emphasis on advice, e.g., keeping active and avoiding bedrest. ^[5]  The PACE study gives useful data for reassurance – half of the participants had recovered from back pain in two-and-a-half weeks regardless of therapy.

Stat Facts

Internal validity and confounding

An experiment is “internally valid” if there are sound reasons to believe that a cause-effect relationship really is present between the independent and dependent variables. ^[6]  Confounding means that the results might be explained by factors other than the experiment’s hypothesis.

References

1. Machado GC, Maher CG, Ferreira PH, Pinheiro MB, Lin CW, Day RO, et al. Efficacy and safety of paracetamol for spinal pain and osteoarthritis: systematic review and meta-analysis of randomised placebo controlled trials. BMJ 2015;350:h1225. DOI: 10.1136/bmj.h1225
2. Roelofs PDDM, Deyo RA, Koes BW, Scholten RJPM, van Tulder MW. Non-steroidal anti-inflammatory drugs for low back pain. Cochrane Database of Systematic Reviews 2008, Issue 1. Art. No.: CD000396. DOI: 10.1002/14651858.CD000396.pub3
3. Williams CM, Maher CG, Latimer J, McLachlan AJ, Hancock MJ, Day RO, et al. Efficacy of paracetamol for acute low-back pain: a double-blind, randomised controlled trial. Lancet 2014;384(9954):1586-96. DOI: 10.1016/S0140-6736(14)60805-9
4. RCT Appraisal Sheets [website]. Centre for Evidence Based Medicine. Retrieved on: 2015 April 16. Available from: http://www.cebm.net/critical-appraisal/
5. Maher CG, Williams C, Lin C, Latimer J. Managing low back pain in primary care. Australian Prescriber 2011(34);5:128-32
6. McBurney DH, White TL. Validity (Chapter 7). In: Research Methods. 7^th Belmont: CA: Thomson Wadsworth, 2007. p. 169-88.