Abstract: We added four studies for the 2019 update. The review now includes 38 trials, recruiting a total of 10,377 participants. Studies lasted between two months and three years and investigated a range of mucolytics, including N-acetylcysteine, carbocysteine, erdosteine, and ambroxol, given at least once daily. Many studies did not clearly describe allocation concealment, and we had concerns about blinding and high levels of attrition in some studies. The primary outcomes were exacerbations and number of days of disability. Results of 28 studies including 6723 participants show that receiving mucolytics may be more likely to be exacerbation-free during the study period compared to those given placebo (Peto odds ratio (OR) 1.73, 95% confidence interval (CI) 1.56 to 1.91; moderate-certainty evidence). However, more recent studies show less benefit of treatment than was reported in earlier studies in this review. The overall number needed to treat with mucolytics for an average of nine months to keep an additional participant free from exacerbations was eight (NNTB 8, 95% CI 7 to 10). High heterogeneity was noted for this outcome (I² = 62%), so results need to be interpreted with caution. The type or dose of mucolytic did not seem to alter the effect size, nor did the severity of COPD, including exacerbation history. Longer studies showed smaller effects of mucolytics than were reported in shorter studies. Mucolytic use was associated with a reduction of 0.43 days of disability per participant per month compared with use of placebo (95% CI -0.56 to -0.30; studies = 9; I² = 61%; moderate-certainty evidence). With mucolytics, the number of people with one or more hospitalisations was reduced, but study results were not consistent (Peto OR 0.68, 95% CI 0.52 to 0.89; participants = 1788; studies = 4; I² = 58%; moderate-certainty evidence). Investigators reported improved quality of life with mucolytics (mean difference (MD) -1.37, 95% CI -2.85 to 0.11; participants = 2721; studies = 7; I² = 64%; moderate-certainty evidence). However, the mean difference did not reach the minimal clinically important difference of -4 units, and the confidence interval includes no difference. Mucolytic treatment was associated with a possible reduction in adverse events (OR 0.84, 95% CI 0.74 to 0.94; participants = 7264; studies = 24; I² = 46%; moderate-certainty evidence), but the pooled effect includes no difference if a random-effects model is used. Several studies that could not be included in the meta-analysis reported high numbers of adverse events, up to a mean of five events per person during follow-up. There was no clear difference between mucolytics and placebo for mortality, but the confidence interval is too wide to confirm that treatment has no effect on mortality (Peto OR 0.98, 95% CI 0.51 to 1.87; participants = 3527; studies = 11; I² = 0%; moderate-certainty evidence). In participants with chronic bronchitis or COPD, we are moderately confident that treatment with mucolytics leads to a small reduction in the likelihood of having an acute exacerbation, in days of disability per month and possibly hospitalisations, but is not associated with an increase in adverse events. There appears to be limited impact on lung function or health-related quality of life. Results are too imprecise to be certain whether or not there is an effect on mortality. Our confidence in the results is reduced by high levels of heterogeneity in many of the outcomes and the fact that effects on exacerbations shown in early trials were larger than those reported by more recent studies. This may be a result of greater risk of selection or publication bias in earlier trials, thus benefits of treatment may not be as great as was suggested by previous evidence.

Plain-language summary: We looked for studies lasting at least two months, in which it was decided at random whether a person received a mucolytic drug or a placebo. We did not include studies involving children or people with other breathing conditions such as asthma and cystic fibrosis. We found 38 studies to include in our review. These studies included a total of 10,377 adults with COPD or chronic bronchitis. The studies used a variety of mucolytic drugs, including N-acetylcysteine, carbocysteine, and erdosteine and lasted from two months to three years. Mucolytics were taken by mouth between one and three times per day. These studies measured several different outcomes to find out if the drug was useful, including flare-ups, hospital admissions, quality of life, lung function, and side effects. We found that people taking mucolytic drugs were less likely to experience a flare-up compared to those taking placebo. Approximately eight people would need to take the drug for nine months for one extra person to avoid having a flare-up. This result was based on 28 studies involving 6723 people. However, the studies carried out a longer time ago (1970s to 1990s) show greater benefit than those carried out more recently. Shorter studies also seemed to show more benefit than longer studies. This could be because the newer trials were larger and may be showing that mucolytics are less beneficial than the earlier studies showed. Or it could be that only studies that showed mucolytics as beneficial were published before the 2000s, when there was a push to report all trial results regardless of whether or not they showed benefit. People taking mucolytics had fewer days of disability (i.e. days when they could not do their normal activities) every month, but this was quite a small difference - less than half a day per person per month. They were also approximately one-third less likely to be admitted to hospital, although this result is based on only five studies that provided this information. Study results suggest that mucolytics do not have an important impact on quality of life or lung function. People taking mucolytics did not experience more unwanted side effects than those taking placebo. But we could not be sure about their impact on death during the study period because only 37 deaths occurred amongst the 3527 participants in studies where deaths were measured and reported. We are moderately confident about the results we have presented. Our confidence is reduced by the results from individual studies looking quite different from one another and the mix of older and newer studies that we found. Also, in some cases there were not enough data to be sure whether mucolytics were better or worse than, or the same as, placebo. Mucolytics appear to be useful for reducing flare-ups, days of disability, and hospital admissions in people with COPD or chronic bronchitis, and they do not appear to cause more side effects. However, they do not appear to have much impact on quality of life or lung function, and we could not be sure about their impact on death. This plain language summary is current to April 2019.