Folate Supplementation for Methotrexate Therapy in Patients With Rheumatoid Arthritis: A Systematic Review, Objective To review the evidence for benefits and harms of folate (folic acid or folinic acid) supplementation on methotrexate (MTX) treatment for rheumatoid arthritis (RA), to assess whether or not folate supplementation would reduce MTX toxicity or reduce MTX benefits, and to decide whether a higher MTX dosage is essential. Methods We performed a sensitive search strategy and searched systematically the Medline, Embase, Web of Science and Cochrane Library databases from inception to 2 June 2016. Abstracts from major rheumatology meetings and major trial registers were also searched to retrieve all randomized controlled trials that interested us. Results Seven studies with 709 patients were included. No significant heterogeneity was found between these trials. For RA patients treated with MTX, those supplied with folate were less likely to have elevated transaminase (odds ratio [OR] 0.15; 95% confidence interval [95% CI] 0.10, 0.23 [p < 0.00001]) and gastrointestinal side-effects such as nausea and vomiting (OR 0.71; 95% CI 0.51, 0.99 [p = 0.04]). Folate appeared to promote compliance to MTX as it reduced patient withdrawal compared to placebo (OR 0.29; 95% CI 0.21, 0.42 [p < 0.00001]). There was no statistical difference for mouth sores between folate and placebo (OR 0.83; 95% CI 0.57, 1.22 [p = 0.35]). As the markers of disease activity in those trials were not consistent, it was impossible to decide whether folate supplementation reduced MTX efficacy. Besides, we compared high-dose folate (≥25 mg per week) and low-dose folate (≤10 mg per week) on MTX efficacy, finding no statistical difference (OR 2.07; 95% CI 0.81, 5.30 [p = 0.13]), nor on MTX toxicity (OR 1.56; 95% CI 0.80,3.04 [p = 0.19]). Conclusion Folate supplementation can reduce the incidence of hepatotoxicity and gastrointestinal side-effects of MTX in patients with RA. It can also reduce patient withdrawal from MTX treatment. Although it tended to reduce mouth sores, it had no statistical significance. No significant difference was found between high-dose folate and low-dose folate on MTX efficacy or toxicity.