H2-blockers: Onset time

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 Antagonistsi.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 within30 minutes if given IV, and within 1 hour if given PO. By either route theduration of action is 10-12 hours. H2 Blockersdo notalter the pH of gastric fluid already present within the stomach at the time of their administration.

Routine premedication with H2 blockers (for aspiration prophylaxis) is not recommended for healthy patients.

Gastric acid secretion involves hydrogen ion secretion by parietal cells in the stomach. It is regulated by acetylcholine, histamine, and gastrin. Suppressing acid secretion can be achieved by blocking histamine receptors in the stomach (H2 receptors) or blocking the H+, K+-ATPase pump (proton pump). Drugs that do the latter are called proton pump inhibitors (PPI’s). Examples include esomeprazole (Nexium) and lansoprazole (Prevacid). Drugs that do the former are called H2 blockers, and examples include cimetidine (Tagamet), ranitidine (Zantac), and famotidine (Pepcid).

Proton pump inhibitors are prodrugs that are activated in an acidic environment. Once the activated drug irreversibly inactivates the H+, K+ ATPase molecule (proton pump), it takes 24 hours to regenerate new pumps to insert into the luminal membrane of the stomach. Thus, PPI’s provide 24-48 hours of acid suppression.

H2 blockers, on the other hand, reversibly compete with histamine for H2 receptors on parietal cells. They increase gastric fluid pH by blocking histamine from inducing gastric acid secretion. They do not reliably alter gastric fluid volume or gastric emptying time. After oral administration, peak serum concentration is within 1-3 hours. Onset of action is rapid after IV dosing, and duration of action after IV dosing are 4-5 hours for cimetidine, 6-8 hours for ranitidine, and 10-12 hours for famotidine. They suppress 24-hour acid secretion by 70%.