Acetaminophen Is Ineffective for Low Back Pain

Authors: Machado GC et al., BMJ 2015 Mar 31; 350:h1225

Its effectiveness for osteoarthritis-related hip and knee pain is marginal.

Acetaminophen is recommended as a first-line analgesic for low back pain and pain related to hip and knee osteoarthritis (OA), but evidence supporting this practice is weak. In this meta-analysis of data from 13 randomized trials, researchers assessed the efficacy and safety of acetaminophen (3000–4000 mg daily) versus placebo in 3500 patients with knee or hip OA and in 1800 patients with low back pain.
In patients with low back pain, acetaminophen was no more effective than placebo for pain and disability in the immediate term (≤2 weeks) or for pain, disability, and quality of life in the short term (>2 weeks–3 months). Patients with hip or knee OA who took acetaminophen had less immediate-term pain (mean difference, −3.3 mm on a 0–100 mm visual analogue scale) but not disability, and less short-term pain (mean difference, −3.7 mm) and disability (mean difference, −2.9 mm) than those who took placebo. Adverse events, treatment withdrawals, medication adherence, and use of rescue medications (e.g., naproxen) did not differ between groups, but acetaminophen users were more likely to have abnormal results on liver function tests (roughly 6% vs. 2%).

Comment
In this meta-analysis, acetaminophen was not effective in patients with low back pain. Although differences in pain and disability in patients with OA-related hip or knee pain were statistically significant, those effects were small (<4 mm difference on a 0–100 mm scale) and probably not clinically meaningful (a difference of ≥9 mm generally is considered to be clinically important). Notably, the long-term efficacy and safety of acetaminophen in patients with back pain or OA-related hip or knee pain are unknown.