Antacids

Antacids neutralize gastric acidity. They are used to relieve the symptoms of dyspepsia and gastro-oesophageal reflux. They promote ulcer healing but less effectively than other therapies.

Aluminium and magnesium-containing antacids

Neither is absorbed from the gut significantly and due to their relatively low water solubility they are long-acting providing that they remain in the stomach. Aluminium-containing antacids have a slower action and produce constipation, while magnesium-containing antacids produce diarrhoea. Aluminium ions form complexes with some drugs (e.g. tetracycline) and reduce their absorption.

Sodium bicarbonate and sodium citrate

These antacids are water-soluble and their onset of action is faster than the aluminium- and magnesium-containing antacids. They are absorbed into the systemic circulation and may cause a metabolic alkalosis if taken in excess. Sodium bicarbonate releases carbon dioxide as it reacts with gastric acid, resulting in belching. Thirty milliliters of 0.3 m sodium citrate is often used with ranitidine to reduce gastric acidity before caesarean section. It should be given less than 10 minutes before the start of surgery due to its limited duration of action.

Drugs influencing gastric secretion

Physiology

Gastrin and ACh stimulate parietal cells (via gastrin and muscarinic receptors) to secrete H+ into the gastric lumen. ACh is released from parasympathetic postganglionic fibres while gastrin is released from G-cells in the antral mucosa. However, the main stimulus for parietal cell acid secretion is via histamine receptor activation.