Abstract: We included in the review 37 studies representing 6128 adult and adolescent participants (most with mild to moderate asthma). Investigators in these studies used three leukotriene receptor antagonists (LTRAs): montelukast (n = 24), zafirlukast (n = 11) and pranlukast (n = 2); studies lasted from four weeks to five years. Anti-leukotrienes and ICS versus same dose of ICS Of 16 eligible studies, 10 studies, representing 2364 adults and adolescents, contributed data. Anti-leukotriene agents given as adjunct therapy to ICS reduced by half the number of participants with exacerbations requiring oral corticosteroids (risk ratio (RR) 0.50, 95% confidence interval (CI) 0.29 to 0.86; 815 participants; four studies; moderate quality); this is equivalent to a number needed to treat for additional beneficial outcome (NNTB) over six to 16 weeks of 22 (95% CI 16 to 75). Only one trial including 368 participants reported mortality and serious adverse events, but events were too infrequent for researchers to draw a conclusion. Four trials reported all adverse events, and the pooled result suggested little difference between groups (RR 1.06, 95% CI 0.92 to 1.22; 1024 participants; three studies; moderate quality). Investigators noted between-group differences favouring the addition of anti-leukotrienes for morning peak expiratory flow rate (PEFR), forced expiratory volume in one second (FEV1), asthma symptoms and night-time awakenings, but not for reduction in β2-agonist use or evening PEFR. Anti-leukotrienes and ICS versus higher dose of ICS Of 15 eligible studies, eight studies, representing 2008 adults and adolescents, contributed data. Results showed no statistically significant difference in the number of participants with exacerbations requiring oral corticosteroids (RR 0.90, 95% CI 0.58 to 1.39; 1779 participants; four studies; moderate quality) nor in all adverse events between groups (RR 0.96, 95% CI 0.89 to 1.03; 1899 participants; six studies; low quality). Three trials reported no deaths among 834 participants. Results showed no statistically significant differences in lung function tests including morning PEFR and FEV1 nor in asthma control measures including use of rescue β2-agonists or asthma symptom scores. Anti-leukotrienes and ICS versus tapering dose of ICS Seven studies, representing 1150 adults and adolescents, evaluated the combination of anti-leukotrienes and tapering-dose of ICS compared with tapering-dose of ICS alone and contributed data. Investigators observed no statistically significant difference in % change from baseline ICS dose (mean difference (MD) -3.05, 95% CI -8.13 to 2.03; 930 participants; four studies; moderate quality), number of participants with exacerbations requiring oral corticosteroids (RR 0.46, 95% CI 0.20 to 1.04; 542 participants; five studies; low quality) or all adverse events (RR 0.95, 95% CI 0.83 to 1.08; 1100 participants; six studies; moderate quality). Serious adverse events occurred more frequently among those taking anti-leukotrienes plus tapering ICS than in those taking tapering doses of ICS alone (RR 2.44, 95% CI 1.52 to 3.92; 621 participants; two studies; moderate quality), but deaths were too infrequent for researchers to draw any conclusions about mortality. Data showed no improvement in lung function nor in asthma control measures. For adolescents and adults with persistent asthma, with suboptimal asthma control with daily use of ICS, the addition of anti-leukotrienes is beneficial for reducing moderate and severe asthma exacerbations and for improving lung function and asthma control compared with the same dose of ICS. We cannot be certain that the addition of anti-leukotrienes is superior, inferior or equivalent to a higher dose of ICS. Scarce available evidence does not support anti-leukotrienes as an ICS sparing agent, and use of LTRAs was not associated with increased risk of withdrawals or adverse effects, with the exception of an increase in serious adverse events when the ICS dose was tapered. Information was insufficient for assessment of mortality.

Plain-language summary: We found 37 studies (representing 6128 adults and adolescents). The people in these trials had mild to moderate asthma. Most (24) studies used the LTRA called montelukast, 11 studies used zafirlukast and only two studies used pranlukast. We divided all studies into three categories to help us make sense of the evidence. • Anti-leukotrienes and ICS versus same dose of ICS: Ten studies (representing 2364 adults and adolescents) contributed data for analysis. Anti-leukotrienes given with ICS reduced by half the number of patients with exacerbations requiring oral steroids (from 9% to 5% over three months), but we are unsure about effects of this treatment on quality of life or serious side effects. Anti-leukotrienes given with ICS improved lung function and asthma control measures. • Anti-leukotrienes and ICS versus higher dose of ICS: Eight studies (representing 2008 adults and adolescents) contributed data for analysis. Results showed no reduction in the number of patients with exacerbations requiring oral steroids and no difference in quality of life nor in side effects. Data showed no improvement in lung function nor in asthma control measures. • Anti-leukotrienes and gradual reduction of ICS dose versus gradual reduction of ICS dose alone: Seven studies (representing 1150 adults and adolescents) evaluated anti-leukotrienes given with a gradually reduced dose of ICS compared with a gradually reduced dose of ICS without use of anti-leukotriene agents. This approach was not beneficial for % reduction in the amount of ICS over time. More people receiving anti-leukotriene and ICS compared with ICS alone experienced increased serious side effects and showed no improvement in lung function nor in asthma control measures. For adolescents and adults with asthma not controlled with daily low-dose ICS, adding anti-leukotriene agents to ICS reduced by half the number of patients with asthma exacerbations requiring an oral corticosteroid. Anti-leukotrienes and ICS also improved lung function and asthma control. However, we are not sure whether the combination of anti-leukotrienes and ICS is superior to higher-dose ICS. Limited available evidence does not support use of anti-leukotrienes as a way to decrease ICS dose. In general, addition of anti-leukotrienes to ICS therapy was not associated with increased side effects, if the dose of ICS was maintained. Our confidence in the evidence was moderate or low for most outcomes.