ObjectiveTo evaluate contextual effects in the form of placebo responses (PRs) for patient-reported pain and function and objectively measured function in osteoarthritis (OA) clinical trials.MethodsTwo authors independently searched major electronic databases from inception to 20 May 2019. Included studies were randomized, placebo-controlled OA trials of pharmacological agents reporting both patient-reported and objectively measured outcomes. PRs for each type of outcome measure were compared by standardized mean differences (SMDs). The placebo response ratio (PRR) assessed the placebo to treatment effect size. The effect sizes of PRs and PRRs were pooled using a random effects model.ResultsTwenty-one trials met the inclusion criteria; 20 were double-blinded with one not reporting on blinding status. Compared with patients’ self-reported outcome (PRO) pain, PRs were significantly lower for PRO function (SMD − 0.16 [95% CI = − 0.28, − 0.05], p = 0.006), objectively measured muscle strength (SMD − 0.34 [95% CI − 0.58, − 0.10], p = 0.006), and range of motion (SMD = − 0.31 [95% CI = − 0.54, − 0.08], p = 0.008) function. Generally, PRs for function outcomes (patient-reported and objectively measured) were similar. The overall PRR for different measures ranged from the smallest (most favorable) for walking time/distance (0.30, 95% CI 0.16 to 0.43) to the largest for PRO pain (0.44, 95% CI 0.23 to 0.65).ConclusionFunction measures both subjective and objective had less contextual effects than pain measures in OA trials. Our results support the OMERACT-OARSI recommendations to include measures of physical function in all clinical trials of hip and knee OA and suggest that a greater use of function measures might enhance the success rates of pharmacological OA trials. Increasing the availability of mobile health apps should facilitate the acquisition of measured function data.