Gastrointestinal/H2 antagonists & antacids

H2 antagonists & antacids/alginates

also: famotidine · ranitidine · Gaviscon · antacid · alginate

Overview

Step-down / lower-potency acid control — H2-receptor antagonists and OTC antacids/alginates for mild reflux and dyspepsia.

Mechanism

H2 antagonists block histamine H2 receptors on parietal cells → reduce acid secretion (less profound than PPIs). Antacids neutralise existing acid; alginates form a floating raft that reduces reflux.

Indications

Mild GORD / dyspepsia
Symptomatic relief (antacid/alginate)
Acid suppression when PPI not suitable

The agents

FamotidineH2 antagonist

Ranitidine WITHDRAWN (NDMA contamination, 2020) → famotidine.

Antacids (calcium/magnesium/aluminium salts)neutraliser

Magnesium → diarrhoea; aluminium → constipation; affect other drug absorption.

Alginates (Gaviscon)raft

Post-meal/bedtime reflux.

Adverse effects

Antacids alter absorption of other drugscommon

Separate dosing (e.g. from levothyroxine, bisphosphonates, tetracyclines).

H2 antagonists: generally well toleratedcommon

Cautions & contraindications

Antacids close to other oral drugsall

Chelation/absorption.

Interactions

Antacids ↓absorption of many drugs; cimetidine (largely obsolete) was a CYP inhibitor + caused gynaecomastia

Monitoring & kinetics

Symptom response; review for alarm features

Oral. Lower potency than PPIs — step-down/OTC tier.

Source: NICE CG184 — Dyspepsia · BNF — Dyspepsia & GORD