Warfarin and antibiotics: why a short course can change your INR

Warfarin has a narrow window. Too little leaves a clot risk; too much raises bleeding risk. The NHS says the INR blood test checks that you are on the right dose and that your dose may change based on the results. Anything that nudges the INR can matter, and antibiotics are among the most frequent culprits.

The NHS warfarin page lists antibiotics among the medicine types that may not mix well with warfarin, along with non-steroidal anti-inflammatories, antidepressants, medicines for heart problems, medicines for stomach ulcers and fungal infections, and statins. That list is deliberately broad. Within it, individual antibiotics differ in how strongly and in which direction they act.

The BNF interaction list for warfarin is the reference prescribers use. It records that several antibiotics increase the anticoagulant effect, which can push the INR above target. For the following, the entry advises monitoring INR, and for some also adjusting the dose.

• Macrolides: clarithromycin and erythromycin increase the anticoagulant effect, with advice to monitor INR and adjust dose.

• Quinolones: ciprofloxacin and levofloxacin increase the effect, with advice to monitor INR.

• Metronidazole: increases the effect, with advice to monitor INR and adjust dose. The NHS metronidazole page also lists warfarin among the medicines to check with a pharmacist or doctor.

• Tetracyclines: doxycycline and tetracycline increase the effect, with advice to monitor INR.

• Penicillins: amoxicillin and flucloxacillin potentially alter the effect, with advice to monitor INR and adjust dose.

• Trimethoprim is predicted to increase the effect; the BNF notes that the manufacturer makes no specific recommendation, so ask your clinic how they would like to handle it.

The BNF entries for these antibiotics do not ask people to avoid them. They state that the antibiotic increases, or may alter, the anticoagulant effect of warfarin and advise monitoring the INR, sometimes with dose adjustment. In other words, the safety net is testing.

An unexpected INR result during an infection is therefore not necessarily a mistake in how you took your tablets. Clinics are used to handling antibiotic courses and will usually want to know about them as soon as they are prescribed.

Say you take warfarin at every consultation, including pharmacies, urgent treatment centres, dentists and out-of-hours services, where the person may not have your record. Show your anticoagulant alert card, which the NHS says to carry and show to doctors or dentists.

Then let your anticoagulation clinic know the name, start date and expected end date. They will usually arrange an earlier INR check, often within a few days, and another after the course ends. If you manage your own INR at home, which the NHS notes is sometimes possible, test more often during this period as advised.

Keep taking warfarin at the usual time unless told otherwise. The NHS metronidazole page says to carry on until you have finished the course even if you feel better, because stopping early could let the infection come back.

The NHS lists blood in pee or poo, unexplained bruising or nosebleeds, coughing up or vomiting blood, and a sudden severe headache with confusion, slurred speech or difficulty moving as possible signs of serious bleeding. It advises calling NHS 111 for bleeding symptoms or a head injury while on warfarin.

An INR that is too high may produce no symptoms at all, which is why testing matters. Gum bleeding, heavier periods, or nosebleeds that last longer than usual are worth reporting to the clinic promptly. For severe or uncontrolled bleeding, call 999.

Prescribers sometimes choose an antibiotic with a smaller effect on warfarin where there is a suitable option, and may shorten the course to what is needed. Topical or single-dose treatments may be preferable in some situations. This is a clinical judgement based on the infection, its severity and local guidance, so it is best to explain your warfarin and let the prescriber decide.

Private and NHS prescribers use the same references, but they may not share records automatically. Giving every clinician the same up-to-date list of medicines is the best protection.