Alginates and antacids

Key Examples

• Gaviscon ©
• Peptac ©

 

Common indications

1. Gastro-oesophageal reflux disease: for symptomatic relief of heartburn.
2. Dyspepsia: for short-term relief of indigestion.

 

Mechanisms of action

• These drugs are most often taken as compound preparations containing an alginate with one or more antacids, such as sodium bicarbonate, calcium carbonate, magnesium or aluminium salts. Antacids work by buffering stomach acids. Alginates act to increase the viscosity of the stomach contents, which reduces the reflux of stomach acid into the oesophagus. After reacting with stomach acid they form a floating ‘raft’, which separates the gastric contents from the gastro-oesophageal junction to prevent mucosal damage. There is some evidence to suggest they also inhibit pepsin production.
• Antacids alone (usually aluminium or magnesium compounds) can be used for the short-term relief of dyspepsia.

 

Important adverse effects

• Compound alginates cause few side effects, which vary depending on their constituents and the dose taken. Magnesium salts can cause diarrhoea, whereas aluminium salts can cause constipation.

 

Warnings

• Compound alginates are well tolerated and are safe in pregnancy. Paediatric formulations are safe for use in infants, but compound alginates should not be given in combination with thickened milk preparations as they can lead to excessively thick stomach contents that cause bloating and abdominal discomfort. Sodium- and potassium-containing preparations should be used with caution in patients with fluid overload or hyperkalaemia (e.g. renal failure). Some preparations contain sucrose, which can worsen hyperglycaemia in people with diabetes mellitus.

 

Important interactions

• The divalent cations in compound alginates can bind to other drugs, reducing their absorption. Antacids can reduce serum concentrations of many drugs, so the doses should be taken at different times. This applies to ACE inhibitors, some antibiotics (e.g. cephalosporins, ciprofloxacin and tetracyclines), bisphosphonates, digoxin, levothyroxine and proton pump inhibitors. By increasing the alkalinity of urine, antacids can increase the excretion of aspirin and lithium.