Fig. 2. Effect of placebo (closed symbols ) and rifampicin (open symbols ) administered before treatment (600 mg by mouth per day for 5 days) on S-ketamine (A ) and S-norketamine (B ) concentrations during and after a 2-h S-ketamine infusion (t = 0–120 min; dose = 20 mg/h). Values are mean ± SEM. Data are from Noppers et al.  11Average responses of the influence of rifampicin or placebo pretreatment on pain intensity (C ), pain appreciation (D ), cognitive flexibility (E ), and reaction time (F ). The responses were measured during and 3 h after a 2-h S-ketamine infusion of 20 mg/h from t = 0 to t = 120 min. Gray squares  are the placebo infusion data after a 5-day pretreatment with placebo or rifampicin (rifampicin-placebo or placebo-placebo). Black circles  are the ketamine infusion data after a 5-day pretreatment with placebo. Open squares  are the ketamine infusion data after a 5-day pretreatment with rifampicin. Values are mean ± SEM. VAS = visual analog score.