Objective The aim of this study was to determine the smallest worthwhile effects of two treatments for nonspecific low back pain (LBP). Study Design and Setting The benefit-harm trade-off method was used to estimate the smallest worthwhile effect of nonsteroidal anti-inflammatory drugs (NSAIDs) and physiotherapy for LBP. Patients seeking care for chronic LBP were interviewed by telephone before treatment commenced and 4 weeks later. Results Patients need to see a median of 30% (interquartile range [IQR]: 10-40) more improvement in pain and 20% (IQR: 10-40) more improvement in disability than would occur without intervention to perceive the effect of NSAIDs are worthwhile. They would need to see 20% (IQR: 0-30) more improvement on pain and disability over natural recovery to perceive that the effect of physiotherapy was worthwhile. There was no difference in estimates of the smallest worthwhile effect elicited at baseline and 4 weeks later. Conclusions People with chronic back pain need to see larger effects on pain of NSAIDS than physiotherapy to consider the effects of these interventions worthwhile. These estimates of the smallest worthwhile effect can be used to interpret the findings of clinical trials and to design adequately powered clinical trials. © 2013 Elsevier Inc. All rights reserved.