Antacids

Antacids are weak bases that neutralise gastric hydrochloric acid, raising intragastric pH within minutes and giving short symptom relief. Alginate-containing products add a floating gel raft that reduces reflux of gastric contents.

Pharmacokinetics. Action lasts about 30 to 60 minutes when taken fasting and up to 3 hours after meals. Magnesium, aluminium and calcium are partly absorbed and cleared by the kidney.

• Aluminium hydroxide: constipating; binds phosphate.
• Magnesium hydroxide or trisilicate: laxative.
• Calcium carbonate 0.5 to 1.5 g chewed as needed: rapid, with acid rebound.
• Sodium bicarbonate: rapid, short, adds a sodium load and gas.
• Sodium alginate with antacid 10 to 20 mL after meals and at bedtime: best for reflux.

Differences. Aluminium and magnesium combinations are used together to balance bowel effects.

• Occasional heartburn and dyspepsia: take 20 to 60 minutes after meals and at bedtime, as needed.
• Reflux in pregnancy: alginate or calcium carbonate preparations first.
• Bridging therapy while a proton pump inhibitor or H2 antagonist takes effect.

Selection. Do not use as sole treatment for persisting symptoms over 2 weeks; investigate.

Contraindications. Hypophosphataemia (aluminium), severe kidney impairment (magnesium and aluminium), hypercalcaemia (calcium), heart failure or hypertension (sodium-rich forms).

Adverse effects. Constipation or diarrhoea; hypermagnesaemia and aluminium accumulation in kidney impairment (encephalopathy, osteomalacia, microcytic anaemia); hypophosphataemia with prolonged aluminium use; milk-alkali syndrome (hypercalcaemia, alkalosis, kidney injury) with excess calcium carbonate.

Interactions. Chelation or raised gastric pH reduces absorption of fluoroquinolones, tetracyclines, levothyroxine, iron, bisphosphonates, itraconazole, atazanavir and rilpivirine. Separate by at least 2 hours; give a fluoroquinolone at least 2 hours before or 6 hours after. Dolutegravir should be taken 2 hours before or 6 hours after any antacid containing aluminium, magnesium or calcium; this matters in people on antiretroviral therapy who self-medicate.

Special groups.

• Pregnancy: calcium and alginate products acceptable; avoid sodium bicarbonate and long-term magnesium trisilicate.
• Breastfeeding: acceptable.
• Children: use only product-specific doses.
• Older adults and kidney impairment: avoid regular aluminium or magnesium.

Monitoring. Calcium, phosphate and magnesium with regular or high-dose use; serum aluminium in dialysis patients.

Referral and escalation. Dysphagia, weight loss, bleeding, anaemia, persistent vomiting, or new dyspepsia over 55 need endoscopy.