Quinolone antibiotics and antacids: timing doses so the antibiotic works

Quinolones are a class of antibiotic. Ciprofloxacin is the best known in the UK, and the NHS describes it as a medicine for serious infections, listing urinary tract infections, chest infections, pneumonia and some sexually transmitted infections among related conditions. It comes as tablets and a liquid, among other forms.

The NHS says you can take ciprofloxacin tablets and liquid with or without food, with the exception of dairy products such as milk, cheese and yoghurt, which can affect how the medicine works. This is the first clue to the interaction: certain substances in food and medicines interfere with absorption.

The BNF entries describe the effect in neutral terms: antacids decrease the absorption of oral ciprofloxacin; iron decreases exposure; zinc is predicted to decrease exposure; calcium acetate decreases absorption; sucralfate decreases exposure. Each comes with a manufacturer recommendation to separate administration, and the severity is rated moderate with evidence from studies.

The practical meaning is that if the antibiotic and the antacid are swallowed together, less antibiotic reaches the blood. The infection is then exposed to a lower concentration of drug than intended. For some infections, particularly those described by the NHS as serious, that is a risk worth avoiding with a simple change in timing.

The NHS ciprofloxacin interaction page gives specific numbers. For antacids used for heartburn or indigestion, take ciprofloxacin at least 2 hours before antacids or at least 4 hours after. For iron tablets such as ferrous sulfate or ferrous fumarate, and for calcium and zinc supplements, leave 2 hours in between supplements and your dose of ciprofloxacin tablets or liquid.

The BNF entries agree: antacids, take 2 hours before or 4 hours after; iron, separate by at least 2 hours; zinc, separate by 2 hours; calcium acetate, separate by 2 hours; sucralfate, separate by 2 hours.

• Antacids for heartburn or indigestion: ciprofloxacin at least 2 hours before, or at least 4 hours after.

• Iron supplements: leave at least 2 hours between the two.

• Calcium and zinc supplements: leave 2 hours between them and ciprofloxacin.

• Milk, cheese and yoghurt: the NHS says to avoid them with ciprofloxacin tablets or liquid, so plan meals to suit.

• Sucralfate, a stomach-protecting medicine: the BNF advises separating by 2 hours.

Suppose you take ciprofloxacin at 8am and 8pm. A simple approach is to take an antacid at lunchtime or mid-afternoon, which sits more than 4 hours after the morning dose and more than 2 hours before the evening dose. If you take daily iron or calcium, a midday slot may suit as well. Writing times on a card on the fridge helps.

If your indigestion is frequent enough to need regular remedies, say so when the antibiotic is prescribed. The prescriber or pharmacist may suggest a different timing or a different antibiotic. Do not stop the antibiotic because of the timing difficulty; ask instead.

Make sure you take the whole course unless a clinician tells you otherwise, and avoid skipping doses just to fit around a remedy.

The NHS lists other medicines to tell your doctor about before taking ciprofloxacin: methotrexate, phenytoin, steroids such as prednisolone, theophylline or aminophylline, tizanidine and warfarin. The BNF warfarin list says ciprofloxacin increases the anticoagulant effect of warfarin and advises monitoring INR.

Spacing advice is separate from these. A person on ciprofloxacin and warfarin still needs INR monitoring, in addition to timing the antacid correctly.

If you are not improving, or if you feel worse, contact the prescriber, the GP surgery or NHS 111 for advice. It is also worth mentioning how you have been taking the antibiotic and any antacids, as this can help them work out whether absorption was a factor.

This advice applies in NHS and private settings alike. A pharmacist can often resolve a spacing question in a few minutes.