“There are two known histamine receptors, designated H1 and H2.
H1 receptor antagonists are typically utilized to suppress the body’s histamine-mediated effects in anaphylactoid or anaphylactic reactions.
H2 antagonists are competitive antagonists at the parietal cell H2 receptor and are typically used to suppress gastric acid secretion. H2 blockers begin working within an hour and last for up to 12 hours.
PPIs are also used for this purpose, but they have a delayed onset of action and work for a longer period of time; most up to 24 hours and the effects may last up to three days.
H2 Antagonists i.e. H2 Blockers, competitively inhibit the action of histamine at H2 receptors on gastric parietal cells, thereby inhibiting gastric acid secretion. The onset of action is within 30 minutes if given IV, and within 1 hour if given PO. By either route, the duration of action is 10-12 hours. H2 Blockers do not alter the pH of gastric fluid already present within the stomach at the time of their administration.”
** H1 histamine blockers are divided into 1st generation and 2nd generation H1 blockers.
Source: https://www.openanesthesia.org/h2-blockers_onset_time/