Aminopyrine is metabolized by cytochrome P450 (CYP) in the liver. The investigators evaluated influences of different PPIs on CYP activity as assessed by the [13C]-aminopyrine breath test ([13C]-ABT). Subjects were 15 healthy volunteers with different CYP2C19 status (5 rapid metabolizers [RMs], 5 intermediate metabolizers [IMs], and 5 poor metabolizers [PMs]). Breath samples were collected before and every 15 to 30 minutes for 3 hours after oral ingestion of [13C]-aminopyrine 100 mg on day 8 of each of the following regimens: control; omeprazole 20 mg and 80 mg, lansoprazole 30 mg, and rabeprazole 20 mg. Changes in carbon isotope ratios in carbon dioxide (13CO2/12CO2) in breath samples were measured by infrared spectrometry and expressed as delta-over-baseline (DOB) ratios (%0). Mean areas under the curve of DOB from 0 to 3 h (AUC0-3h of DOB) were significantly decreased by omeprazole 20 mg and lansoprazole 30 mg but not by rabeprazole 20 mg. Conversely, higher PPI dose (ie, omeprazole 80 mg) seemed to further decrease AUC0-3h of DOB in RMs but increased it in PMs. Omeprazole and lansoprazole at the standard doses inhibit CYP activity but rabeprazole does not, whereas high-dose omeprazole seems to induce CYPs.