Abstract: We included eight RCTs (with a total of 559 participants) in this review. The studies recruited outpatients aged between 17 and 76 years who were referred to endoscopy centres in several different countries. The certainty of evidence was reduced for most outcomes due to the poor methodological quality of included studies; issues mainly related to the generation of allocation sequence, allocation concealment, and blinding (this last domain related specifically to adverse outcomes). We are uncertain whether the addition of NAC to antibiotics improves H pylori eradication rates, compared with the addition of placebo or no NAC (38.8% versus 49.1%, risk ratio (RR) 0.74, 95% confidence interval (CI) 0.51 to 1.08; participants = 559; studies = eight; very low-certainty evidence). A post-hoc sensitivity analysis, in which we removed studies that tested antibiotic regimens no longer recommended in clinical practice, showed that the addition of NAC may improve eradication rates compared to control (27.2% versus 37.6%, RR 0.71, 95% CI 0.53 to 0.94; participants = 397; published studies = five). We are uncertain whether NAC is associated with a higher risk of gastrointestinal adverse events compared to control (23.9% versus 18.9%, RR 1.25, 95% CI 0.85 to 1.85; participants = 336; studies = five; very low-certaintyevidence), or allergic adverse events (2% versus 0%, RR 2.98, 95% CI 0.32 to 27.74; participants = 336; studies = five; very low-certainty evidence). There were no reports of toxic adverse events amongst included studies. We are uncertain whether the addition of NAC to antibiotics improves H pylori eradication rates compared with the addition of placebo or no NAC. Due to the clinical, statistical and methodological heterogeneity found in included studies, and the uncertainty observed when analysing therapy subgroups, any possible beneficial effect of NAC should be regarded cautiously. We are uncertain whether NAC is associated with a higher risk of gastrointestinal or allergic adverse events compared with placebo or no NAC. There were no reports of toxic adverse events amongst the included studies. Further large, well-designed, randomised clinical studies should be conducted, with good reporting standards and appropriate collection of efficacy and safety outcomes, especially for current recommended antibiotic regimens.

Plain-language summary: We included eight studies (specifically, randomised controlled trials (RCTs)) with a total of 559 people aged between 17 and 76 years old. The evidence is current to April 2018. All studies recruited outpatients from endoscopy centres (centres that specialise in an examination done with a flexible tube with a camera that is inserted into stomach) in several countries. The antibiotic combinations tested were very different in the included studies, as were the doses of NAC (600 mg to 1800 mg per day). NAC was compared with placebo (dummy pill) or nothing. We are uncertain whether the addition of NAC to antibiotics improves H pylori cure rates compared with the addition of placebo or no NAC. Any possible beneficial effect of NAC should be regarded cautiously because the included studies were very different and of low certainty, with some flaws that could have compromised their results and consequently, the results of this review. We are uncertain whether NAC is associated with a higher risk of gastrointestinal or allergic adverse events compared with placebo or no NAC. There were no reports of toxic adverse events amongst the included studies. Further large, well-designed randomised clinical studies, with good reporting standards and appropriate collection of effectiveness and safety outcomes should be done, especially for current recommended antibiotic combinations. The overall certainty of the evidence for eradication rates ranged from very low to low. Five studies provided information on adverse events (side effects), and the certainy of evidence was very low. The included studies were poorly conducted and this reduced our confidence in the results.