Concurrent administration of phenobarbital (PB) and omeprazole (OME) results in a drug-drug interaction and reduces omeprazole efficacy. (A) An illustration of animal treatment. Adult mice in the OME-control group (n = 3) were treated with three consecutive doses of 150 mg/kg per day of omeprazole at 57, 58, and 59 days after birth. Adult mice in the PB/OME cotreatment group (n = 3) received a single dose of 200 mg/kg phenobarbital together with a dose of 150 mg/kg omeprazole at day 57, followed by 2 days of treatment with just omeprazole at days 58 and 59. Adult mice in the PB/OME post-treatment group (n = 3) received the same single dose of 200 mg/kg phenobarbital at day 57, then the three consecutive doses of 150 mg/kg per day of omeprazole were started 3 days later at days 60, 61, and 62. (B) Gastric pH with mean ± S.D. in the mouse stomach at 1 hour after the last treatment of omeprazole. (C) Relative fold changes of mRNA of CYP2C29, (D) mRNA of CYP3A11, and (E) protein of CYP3A11 in the mouse livers with PB (control, co-, and post-)/OME treatment. ***p < 0.001.