INTRODUCTION: Bowel preps for colonoscopy can cause transient changes in gut microbiota. Rifaximin is indicated as IBS-D treatment in adults. This post hoc analysis evaluated effect of time since colonoscopy on rifaximin response for IBS-D. METHODS: Two phase 3 studies (TARGET 1/2) comprised adults who had documented history of colonoscopy <2 y prior to enrollment for IBS evaluation; otherwise they had 1 within 30 d of signed consent. Patients (pts) were randomized to rifaximin 550 mg TID or placebo for 2 wks. Responders, grouped by time from colonoscopy to treatment start (≤60 d or >60 d), were analyzed using LOCF. Composite responders (FDA endpoint) were pts meeting weekly response criteria for abdominal pain (≥30% decrease from baseline in mean weekly pain score [range, 0-6]) and stool consistency (≥50% decrease from baseline in d/wk with Bristol Stool Scale type 6/7 stool) during ≥2 of first 4 wks posttreatment; responders for individual components also were determined. RESULTS: Differences in responder rates between rifaximin and placebo were observed in colonoscopy-timing subgroups (≤60 d or >60 d), with largest differences seen in >60 d group (Figure 1). Among pts who had colonoscopy >60 d prior to starting treatment in TARGET 1, a greater percentage treated with rifaximin (n=109) vs placebo (n=106) were composite (47.7% vs 34.0%; P = 0.04), abdominal pain (53.2% vs 40.6%; P = 0.06), or stool consistency (81.7% vs 63.2%; P = 0.002) responders. For colonoscopy ≤60 d prior to starting treatment, a greater percentage of pts treated with rifaximin (n = 199) vs placebo (n = 208) were composite (45.7% vs 40.9%; P = 0.32), abdominal pain (50.3% vs 42.8%; P = 0.13), or stool consistency (77.4% vs 69.7%; P = 0.08) responders. In TARGET 2, a greater percentage of pts treated with rifaximin (n = 106) vs placebo (n = 97) in colonoscopy >60 d prior group were composite (45.3% vs 32.0%; P = 0.05), abdominal pain (53.8% vs 39.2%; P = 0.04), or stool consistency (71.7% vs 61.9%; P = 0.14) responders. A greater percentage of pts treated with rifaximin (n = 209) vs placebo (n = 223) in colonoscopy ≤60 d prior group were responders, although differences were smaller: composite (47.4% vs 38.1%; P = 0.05), abdominal pain (51.7% vs 44.8%; P = 0.16), and stool consistency (75.1% vs 65.5%; P = 0.03). CONCLUSION: Larger differences in responder rates with rifaximin vs placebo were observed in pts in whom colonoscopy was >60 d from start of IBS-D therapy. This may be related in part to reestablishment of IBS-related gut microbiota dysbiosis; further research is needed.