Digoxin and diltiazem: higher digoxin levels and a slower heartbeat

The NHS describes digoxin as a cardiac glycoside used to control some heart problems, such as irregular heartbeats including atrial fibrillation, and to help manage heart failure symptoms, usually with other medicines. It works by slowing the heart rate and making the heart beat more strongly.

Diltiazem is described by the NHS as a medicine used to treat high blood pressure, to prevent chest pain caused by angina and to reduce the frequency of attacks in Raynaud's phenomenon. It also comes as a cream or ointment for anal fissures. For high blood pressure and angina, it may take a couple of weeks to work fully.

The two can be prescribed in the same person, since someone with a heart rhythm problem may also have blood pressure or angina. The reason for caution is that each affects the heart and one changes the level of the other.

The BNF interaction list for digoxin has an entry for diltiazem saying that diltiazem increases the concentration of digoxin. The manufacturer advises monitor and adjust the dose. The severity is severe and the evidence type is study. The same entry appears from the diltiazem side.

A second statement sits alongside it: both digoxin and diltiazem can increase the risk of bradycardia. That is a different mechanism, an added effect of each medicine on heart rate, so it can matter even if the digoxin level is as intended.

The entries do not say avoid. They describe a combination that is used with attention to the dose and to the heart rate.

The NHS digoxin side effects page says some people are more sensitive to digoxin than others, including children, older people and people with kidney disease or an underactive thyroid. Common effects include feeling dizzy, feeling or being sick, diarrhoea, changes in vision and skin rashes. It says to call a doctor or 111 now if you have more than 2 of the common side effects, as it means you could have too much digoxin in your blood.

It says to call 999 or go to A&E if you have a fast heart rate or palpitations, shortness of breath, or feel dizzy or light-headed, and not to drive yourself. The NHS diltiazem page adds that chest pain which does not stop after a few minutes, or is new or worse if you have angina, needs 999.

This site has a separate overview of digoxin toxicity and the digoxin level test for readers who want more.

• More than two common side effects at once: call your doctor or 111.

• Fast heart rate, palpitations, breathlessness or light-headedness: 999 or A&E.

• Chest pain that does not stop after a few minutes: 999.

If diltiazem is started or stopped in someone taking digoxin, expect the team to think about the digoxin dose and heart rate. The NHS digoxin page says you will need regular blood tests to check that your kidneys are healthy and you have the right amount of potassium, magnesium and calcium. Ask whether a digoxin level is needed after a change.

Do not stop or double either medicine. The NHS also says to ask a pharmacist before using antacids, kaolin or laxatives with digoxin, and not to take NSAIDs such as ibuprofen regularly without checking with your doctor.

Pharmacists check interactions at dispensing in the NHS, and GP practices and hospital clinics review medicines. Private cardiologists use the same BNF, so a shared medicine list is what keeps both services consistent.