Omeprazole is a substituted benzimidazole that suppresses gastric acid secretion by specific inhibition of the gastric acid proton pump (H /K ATPase enzyme) at the secretory surface of the gastric parietal cell. It blocks the final step of acid secretion. After oral administration, the onset of the antisecretory effect of Omeprazole occurs within one hour, with the maximum effect occurring within two hours and the duration of inhibition lasts up to 72 hours. The antisecretory effect lasts far longer than would be expected from the very short (less than one hour) plasma half-life, apparently due to prolonged binding to the parietal H /K ATPase enzyme. Following absorption, Omeprazole is almost completely metabolized and rapidly eliminated mostly through urine.