IT IS COMPETITIVE H2 RECEPTORS ANTAGONIST. IT ACTS TO MARKEDLY DECREASE ACID OUTPUT BY COMPLETELY INHIBITING HISTAMINE INDUCED GASTRIC ACID SECRETION. BOTH THE ACID CONCENTRATION AND VOLUME OF GASTRIC SECRETION ARE SUPPRESSED BY FACID, WHILE CHANGES IN PEPSIN SECRETION ARE PROPORTIONAL TO VOLUME OUTPUT. FACID INHIBITED BASAL AND NOCTURNAL GASTRIC SECRETION, AS WELL AS SECRETION STIMULATED BY FOOD AND PENTAGASTRIN. AFTER ORAL ADMINISTRATION, THE ONSET OF THE ANTISECRETORY EFFECT OCCURRED WITHIN ONE HOUR; THE MAXIMUM EFFECT WAS DOSE-DEPENDENT, OCCURRING WITHIN ONE TO THREE HOURS. DURATION OF INHIBITION OF SECRETION BY DOSES OF 20 AND 40 MG WAS 10 TO 12
HOURS.