Digoxin and verapamil: how the two medicines interact

Digoxin is a cardiac glycoside. The NHS says it is used to control some heart problems, such as irregular heartbeats including atrial fibrillation, and can help manage the symptoms of heart failure, usually with other medicines. It slows the heart rate and makes the heart beat more strongly, which makes it easier for the heart to pump blood around the body.

Verapamil is a calcium channel blocker used to treat irregular heart rhythms and high blood pressure, and to prevent chest pain from angina. The NHS says it works by relaxing and widening blood vessels, and that it also relaxes the heart and slows the heart rate, which helps treat rhythm problems.

Because both slow the heart and both are used for rhythm control, they can overlap in the same person, for example someone with atrial fibrillation whose treatment has been changed over time. That overlap is the reason each NHS page names the other medicine as one to mention to your doctor.

The NHS digoxin page says some medicines can affect the way digoxin works or can increase the risk of side effects, and lists verapamil in the group of heart rhythm and blood pressure medicines to tell your doctor about. The NHS verapamil page lists digoxin among the medicines to mention before starting verapamil, and notes that taking verapamil with other medicines that lower blood pressure can sometimes lower it too much, causing dizziness or fainting.

The pages read for this guide do not state numerical effects, and it would be wrong to put a figure on them here. What the NHS pages do say is that some medicines can affect the way digoxin works or increase the risk of side effects, that some people are more sensitive to digoxin than others, and that blood tests are used to keep track of it. A prescriber who adds or changes a medicine on the list will normally review how you are getting on.

The NHS lists dizziness, feeling or being sick, diarrhoea, changes in vision (including blurred vision and not being able to look at bright light) and skin rashes as common side effects. It adds a specific instruction: tell your doctor if you have more than 2 of the common side effects, because it could mean you have too much digoxin in your blood.

The NHS also says to call your doctor or 111 now in that situation, and to call 999 or go to A&E if you have a fast heart rate (palpitations), shortness of breath, or feel dizzy or light-headed. It adds that you should not drive to A&E, and should bring all your medicines.

• Do not drive, cycle or use tools if you have dizziness or vision problems, as the NHS advises.

• Take the digoxin packet and any other medicines to hospital if you are taking more than your prescribed dose.

• If you have taken more than your prescribed dose, the NHS says to contact 111 for advice now.

The NHS says the most common side effect of verapamil is constipation, and lists feeling sick, flushing, headaches, and feeling dizzy or tired. It advises calling your doctor or 111 straight away if the whites of your eyes or skin turn yellow, if you have dark pee, or if you have swelling in your hands, feet, ankles or legs. Chest pain that does not stop after a few minutes, especially with sweating, sickness, light-headedness or breathlessness, is a reason to call 999.

Two practical points come from the interaction pages. First, the NHS says not to eat grapefruit or drink grapefruit juice while taking verapamil because it can increase the amount of verapamil in the body and make side effects worse. Second, St John's wort is named on both the digoxin and verapamil pages as a herbal remedy that can affect how the medicine works, and the NHS asks you to talk to a pharmacist or doctor before using it.

On the NHS dosing page, digoxin may start with a larger initial dose, with a blood test afterwards to see how it has worked, followed by a daily dose. The usual daily dose for adults is given as 125 micrograms to 250 micrograms, and doses are usually lower for older people and for people with kidney disease because they may be more likely to get side effects. Doses should only ever be set by the prescriber.

Regular blood tests check kidney function and the potassium, magnesium and calcium in your blood, according to the NHS. If a person has taken too much, the NHS says a doctor may ask for a blood test to see how much digoxin is in the blood and an electrocardiogram (ECG) to see the effect on the heartbeat.

If you need an operation, the NHS says to tell your doctor you take digoxin, and they will say whether you need to stop it beforehand.

Digoxin is often taken long term, and the NHS says it is normally recommended when other medicines have not been enough on their own. Stopping suddenly can make the condition worse, and the NHS says the same about verapamil, where stopping may cause blood pressure to rise and increase the risk of heart attack or stroke. If a dose is missed, the NHS says to leave out the missed digoxin dose and take the next at the usual time, never taking two doses together.

Pharmacists can help with over-the-counter products. The NHS digoxin page asks you to ask a pharmacist before using antacids, kaolin or laxatives, and not to take NSAIDs such as ibuprofen regularly without checking with your doctor first. For painkillers on verapamil, the NHS says paracetamol is the best choice.

Digoxin and verapamil are usually prescribed and monitored through the GP surgery, often after a cardiology or heart failure clinic has set the plan. Community pharmacists can review a medicine list at no cost. A private cardiology consultation may offer faster access to ECGs and specialist review, but the practical monitoring, blood tests and prescriptions still need a clinician who has the whole medicines list.