Antacids, iron and calcium: spacing doses so medicines are absorbed

Antacids neutralise stomach acid and contain metal ions such as aluminium, magnesium and calcium. Iron and calcium supplements are also sources of metal ions. Several medicines bind to these ions in the gut, forming complexes that are poorly absorbed, so less of the medicine reaches the blood.

Other medicines need stomach acid to dissolve properly. Either mechanism leads to the same practical result: a tablet that is swallowed but does not do its job. Spacing doses lets each be absorbed before the other arrives.

The NHS page on taking ciprofloxacin with other medicines says to take it at least 2 hours before antacids or at least 4 hours after them. It also says to leave 2 hours between iron, calcium or zinc supplements and your dose, naming ferrous sulfate and ferrous fumarate as examples.

The same page lists methotrexate, phenytoin, steroids such as prednisolone, theophylline or aminophylline, tizanidine and warfarin as medicines to tell the prescriber about. Our theophylline-quinolone guide covers one of those pairings in more depth.

The NHS Gaviscon page says not to take it within 2 hours before or after a list of medicines. That list includes antihistamines, quinolone and tetracycline antibiotics, iron tablets, antifungal medicines, beta blockers, penicillamine, steroids, antipsychotics, chloroquine, bisphosphonates such as alendronic acid and levothyroxine.

Paracetamol is the exception noted by the NHS: it can be taken at the same time, while ibuprofen and aspirin should not be taken with Gaviscon without medical advice. The long list shows why checking before reaching for an indigestion remedy is sensible.

• Quinolone and tetracycline antibiotics.

• Iron tablets and levothyroxine.

• Bisphosphonates such as alendronic acid.

• Antifungals, beta blockers and steroids.

The NHS ferrous sulfate page says it is a good idea to leave a gap between ferrous sulfate and any other medicine, and names antacids, levothyroxine, ciprofloxacin and calcium as examples. It advises asking a doctor or pharmacist how long the gap should be.

For levothyroxine specifically the NHS says to take it on an empty stomach 30 to 60 minutes before the first meal and lists calcium and iron supplements and antacids as interacting. Our levothyroxine guide gives a fuller routine.

The NHS gabapentin page says antacids reduce how much gabapentin is absorbed, and that you should take gabapentin at least 2 hours before or after the antacid. For someone with nerve pain or epilepsy, an unreliable dose may mean breakthrough symptoms, so the spacing matters even though the tablet seems harmless.

Write down every tablet and supplement with the time of day. Put the sensitive medicine, such as levothyroxine or an antibiotic, in its own slot, and move the antacid, iron or calcium to a different time. Use phone reminders if the spacing is awkward.

Ask a pharmacist if your list is long, since a single regimen may need several different gaps. A prescriber can sometimes simplify by changing the timing of the supplement or choosing a different product.