Canadian researchers assessed if language of publication restrictions impact the estimates of an intervention’s effectiveness, if such impact is similar for conventional medicine and complementary medicine interventions, and if the results are influenced by publication bias and statistical heterogeneity. They set out to examine the extent to which including reports of RCTs in languages other than English influences the results of systematic reviews, using a broad dataset of 42 language-inclusive systematic reviews, involving 662 RCTs, including both conventional medicine and CAM interventions. For conventional medicine interventions, language-restricted systematic reviews, compared with language-inclusive ones, did not introduce biased results in terms of estimates of intervention effectiveness (random effects ratio of odds ratio, ROR 1.02; 95% CI 0.83 to 1.26). For CAM interventions, however, language-restricted systematic reviews resulted in a 63% smaller protective effect estimate than language-inclusive reviews (random effects ROR 1.63; 95% CI 1.03 to 2.60).