Felodipine and erythromycin: a blood pressure tablet and a common antibiotic

Felodipine is a calcium channel blocker. The NHS says it is used to treat high blood pressure and angina, that it works by relaxing and widening blood vessels to increase blood flow and the oxygen supply to the heart, and that it comes as a once-a-day tablet taken in the morning, usually long term.

Erythromycin is an antibiotic. The NHS says it is mainly used for bacterial infections, including chest infections, dental abscesses and sexually transmitted infections, and sometimes for skin conditions such as acne and rosacea. The NHS says most people take it 2 to 4 times a day for a few days or weeks.

Because the two are prescribed for very different reasons, they often meet by accident: a person on long-term felodipine for blood pressure picks up a short antibiotic course for a chest infection from a different clinic, an urgent care centre or a dentist.

On the felodipine page, under taking felodipine with other medicines, food and drink, the NHS says some medicines, remedies and supplements may not mix well with it. The list includes antibiotics such as erythromycin, antifungal medicines such as ketoconazole, epilepsy medicines, medicines for stomach ulcers, medicines for HIV, and herbal remedies such as St John's wort. It asks you to speak to a pharmacist or doctor before taking any other medicine, herbal remedy, vitamin or supplement.

The NHS erythromycin page lists a different set of cautions: domperidone for nausea, anticoagulants such as warfarin or rivaroxaban, the mental health medicines pimozide and amisulpride, and statins such as simvastatin and lovastatin. Felodipine is not named on the erythromycin page, which is a reminder that interaction lists are written from each medicine's side and are not complete mirrors of each other.

Neither page explains the mechanism or describes a numerical change, and this guide will not supply one. The practical conclusion is that the combination appears on the felodipine caution list and should be known to the prescriber and pharmacist.

Felodipine already has a familiar set of side effects, and anything that changes how much of it is in the body may make them more noticeable. The NHS lists common side effects of felodipine as gradually swollen ankles, headaches, flushing, stomach pain, dizziness, feeling or being sick and a more noticeable heartbeat or palpitations. It also says felodipine can sometimes cause dizziness and tiredness, and that alcohol can increase risks such as low blood pressure.

Serious side effects listed by the NHS include an allergic reaction, sudden swelling of the face, tongue or genitals (angioedema) and chest pain, with advice to call NHS 111 if you think you are having serious side effects. For erythromycin, the NHS lists severe stomach pain, yellowing of the skin or eyes or pale poo with dark pee, severe or bloody diarrhoea, and says to call 999 for anaphylaxis and 111 for other serious effects.

• New or worse dizziness or light-headedness, particularly when standing up.

• A pounding, fluttering or very fast heartbeat that is new for you.

• More ankle swelling or flushing than usual.

• Any chest pain, which the NHS felodipine page includes among serious side effects.

• Severe or bloody diarrhoea, yellow skin or eyes, or severe stomach pain on erythromycin.

• Swelling of the lips, tongue, face or throat, or difficulty breathing, which needs 999.

Say that you take felodipine whenever an antibiotic is prescribed, and say which antibiotic when you collect it so a pharmacist can check. Keep a current list of medicines, including supplements, and avoid grapefruit and grapefruit juice throughout, as the NHS advises. Do not add St John's wort, which appears on both the felodipine and erythromycin pages.

Keep taking felodipine as prescribed unless a doctor tells you otherwise, since the NHS warns that stopping suddenly can be dangerous. Take the full antibiotic course, as the NHS says it is important to keep taking erythromycin until you have finished it, even if you feel better. If you miss a dose of either medicine, follow the leaflet: the NHS says not to take two doses together to make up for a missed one.

If an antibiotic has been suggested by a pharmacist, urgent treatment centre or private clinician, the same questions apply. They may not have your records, so bring the box of felodipine or a list.

For questions about interactions, a community pharmacist is often the quickest route, and NHS 111 can help out of hours. Your GP surgery can review long-term medicines, and your blood pressure should still be checked at the usual intervals. If you take more than your prescribed dose of felodipine, the NHS says to get help from NHS 111.

Do not stop felodipine because an antibiotic has been started unless a clinician has advised it. Equally, do not refuse a needed antibiotic out of worry; ask for the interaction to be checked and for an alternative if one is appropriate.