Table 3 shows the assessment of risk of bias . We excluded five trials from the meta-analysis as the risk of bias was unclear in four or more of the six domains47 52 60 or because no data were reported in the published papers and the study authors did not respond to our requests for data.48 49 52 This resulted in exclusion of 114 patients (1.9%) from the varenicline group and 123 patients (2.4%) from the placebo group; 10 761 patients were included in the meta-analyses (5817 in the varenicline group and 4944 in the placebo group). In three trials39 41 42 there were high losses to follow-up, 53.7%, 60%, and 45.2%, respectively. These studies were assessed as low risk of bias because of incomplete outcome data and were not excluded. They were all small trials (combined total of n=92 in the varenicline group (1.6% of all people randomised to varenicline in our meta-analysis) and n=86 in the placebo group (1.7% of all people randomised to placebo)). In addition, only nine out of 178 patients (5.1%) were lost to follow-up because of adverse events, and these were mostly gastrointestinal in nature (such as nausea and vomiting).39 42 41
