Effect of nizatidine on paracetamol and its metabolites in human plasma.

The effect of the histamine H2-receptor antagonist, nizatidine, on plasma concentrations of paracetamol has been investigated with respectto hepatic metabolism. Paracetamol (1000 mg) together with 300 or 150 mg nizatidine or placebo was orally administered to five healthy male volunteers. Venous blood samples were taken before and after administration. Plasma paracetamol and paracetamol conjugates (glucuronide and sulfate) were measured by high-performance liquid chromatography. The pharmacokinetic parameters were calculated from the plasma paracetamol concentration-time curves of each volunteer. The plasma nizatidine concentration was highest (2420.0+/-192.4 and 996.0+/-54.6 ng mL(-1)) in the sample taken 1 h after administration of 300 mg nizatidine (high dose) and 150mg nizatidine (low dose), respectively. Plasma paracetamol concentrations with nizatidine (high and low doses) were increased significantly at 45-120 min and 45-60 min, respectively, compared with placebo. The total area under the plasma paracetamol concentration-time curve from 0 to 180 min (2361.5+/-146.4 and 2085.75+/-73.5 microg min mL(-1)) significantly increased after coadministration of nizatidine (high and low doses), respectively (P < 0.01 vs placebo). Paracetamol glucuronide concentrations with nizatidine (high and low doses) were decreased significantly at 30-45 min and 30 min, respectively, compared with placebo. However, plasma paracetamol sulfate concentrations with nizatidine (high and low doses) were not significantly altered. The coadministration of nizatidine (150 and 300 mg) dose-dependently reduces plasma paracetamol glucuronide concentrations and increases plasma paracetamol concentrations. The effects of nizatidine could result from the inhibition of glucuronyltransferase. Thus, care is necessary when paracetamol and nizatidine are coadministered.