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RATIONALE 'Forgiveness' charts the ability of a drug or regimen to withstand non-adherence without negative clinical consequences. OBJECTIVES We aimed to determine the influence of regimen length, regimen drugs and dosing, and when during treatment non-adherence occurs on the forgiveness of anti-tuberculosis regimens. METHODS Using data from three randomised controlled trials comparing experimental four-month regimens for drug-sensitive tuberculosis with the standard six-month regimen, we used generalised linear models to examine how the risk of a negative composite outcome changed as dose-taking decreased. The percentage of doses taken and absolute number of doses missed were calculated, during the intensive and continuation phases of treatment, and overall. A mediation analysis was undertaken to determine how much of the association between intensive phase dose-taking and the negative composite outcome was mediated through continuation phase dose-taking. MEASUREMENTS AND MAIN RESULTS Forgiveness of the four-month and six-month regimens did not differ for any treatment period. Importantly, four-month regimens were no less forgiving of small numbers of absolute missed doses than the six-month regimen (e.g. for 3-7 missed doses versus no missed doses (baseline), six-month regimen adjusted risk ratio 1.65 (95% confidence interval 0.80-3.41) and four-month regimens 1.80 (1.33-2.45)). No four-month regimen was conclusively more forgiving than another. We found evidence of mediation by continuation phase dose-taking on the intensive phase dose-taking and negative composite outcome relationship. CONCLUSIONS With the current appetite for, and progress towards, shorter drug-sensitive tuberculosis regimens worldwide, we offer reassurance that shorter regimens are not necessarily less forgiving of non-adherence. Given the importance of continuation phase adherence, patient support during this period should not be neglected.