Laxative use after cardiac arrest: exposure timing, treatment selection, and outcome interpretation.

We commend Duan et al. for examining laxative use after cardiac arrest. However, because follow-up began at return of spontaneous circulation, whereas exposure timing was not reported, the observed association may reflect both immortal time and treatment selection: patients who survived and recovered sufficiently may have been more likely to receive laxatives. A time-varying exposure defined by first administration, with landmark analysis as a sensitivity analysis, would better align treatment and risk time. The reported time-varying-effect model does not estimate the effect of initiating treatment on a particular day and, therefore, cannot support a 15-day treatment window. The findings should be framed as observational associations rather than evidence that laxatives improve survival.