Abstract: Two RCTs were included, involving 2667 adult patients with RRMS to evaluate the efficacy and safety of two dosages of dimethyl fumarate (240 mg orally three times daily or twice daily) by direct comparison with placebo for two years. Among them, a subsample of 1221 (45.8%) patients were selected to participate in MRI evaluations by each study site with MRI capabilities itself. No powered head-to-head study with an active treatment comparator has been found. Meta-analyses showed that dimethyl fumarate both three times daily and twice daily reduced the number of patients with a relapse (risk ratio (RR) 0.57, 95% confidence interval (CI) 0.50 to 0.66, P < 0.00001 and 0.64, 95% CI 0.54 to 0.77, P < 0.00001, respectively) or disability worsening (RR 0.70, 95% CI 0.57 to 0.87, P = 0.0009 and 0.65, 95% CI 0.53 to 0.81, P = 0.0001, respectively) over two years, compared to placebo. The treatment effects were decreased in the likely-case scenario analyses taking the effect of dropouts into consideration. Both dosages also reduced the annualised relapse rate. Data of active lesions on MRI scans were not combined because there was a high risk of selection bias for MRI outcomes and imprecision of MRI data in both studies, as well as an obvious heterogeneity between the studies. In terms of safety profile, both dosages increased the risk for adverse events and the risk for drug discontinuation due to adverse events. The most common adverse events included flushing and gastrointestinal events (upper abdominal pain, nausea and diarrhoea). Uncommon adverse events included lymphopenia and leukopenia, but they were more likely to happen with dimethyl fumarate than with placebo (high dosage: RR 5.25, 95% CI 2.20 to 12.51, P = 0.0002 and 5.23, 95% CI 2.47 to 11.07, P < 0.0001, respectively; low dosage: RR 5.69, 95% CI 2.40 to 13.46, P < 0.0001 and 6.53, 95% CI 3.13 to 13.64, P < 0.00001, respectively). Both studies had a high attrition bias resulting from the unbalanced reasons for dropouts among groups. Quality of evidence for relapse outcome was moderate, but for disability worsening was low. There is moderate-quality evidence to support that dimethyl fumarate at a dose of 240 mg orally three times daily or twice daily reduces both the number of patients with a relapse and the annualised relapse rate over two years of treatment in comparison with placebo. However, the quality of the evidence to support the benefit in reducing the number of patients with disability worsening is low. There is no high-quality data available to evaluate the benefit on MRI outcomes. The common adverse effects such as flushing and gastrointestinal events are mild-to-moderate for most patients. Lymphopenia and leukopenia are uncommon adverse events but significantly associated with dimethyl fumarate. Both dosages of dimethyl fumarate have similar benefit and safety profile, which supports the option of low-dose administration. New studies of high quality and long-term follow-up are needed to evaluate the benefit of dimethyl fumarate on prevention of disability worsening and to observe the long-term adverse effects including progressive multifocal leukoencephalopathy.

Plain-language summary: We searched medical databases for studies in which participants were randomly assigned to dimethyl fumarate or a control drug (randomised controlled trials). The efficacy of this therapy was considered in terms of occurrence of relapses and progression of the disease. We found moderate quality evidence that both dosages of dimethyl fumarate reduce the number of people with RRMS having relapse after two years of treatment, while there is low quality evidence showing that the medicine reduces the number of people who experience worsening disability at the end of two years.  Common adverse effects such as flushing and gastrointestinal events (diarrhoea, nausea and upper abdominal pain) are mild to moderate for most of patients. Dimethyl fumarate can have an effect on the body's immune system by causing a drop in the number of the white blood cells which help to fight infection. More people in the groups treated with dimethyl fumarate experienced this than they did with placebo. We found moderate quality that people were more likely to leave the study early because of adverse events if they were treated with dimethyl fumarate than with placebo.