Digoxin toxicity: warning signs, causes and what to do

Digoxin is a medicine used to control the heart rate in conditions such as atrial fibrillation and, less often now, heart failure. The British Heart Foundation explains that it slows the heart rate and is usually used after other medicines have been tried or alongside them.

The BHF stresses that the dose is crucial: too little will not help, but too much can cause nausea, vomiting and changes to the electrical system of the heart. That is why people taking it have regular blood tests to keep levels within carefully defined limits.

Digoxin toxicity is the state in which levels, or the effect on the body, have become too high. It can follow an overdose, but it can also develop gradually in someone taking the correct dose when kidney function declines, when other medicines interact, or when blood minerals shift.

The NHS lists the common side effects of digoxin and says that having more than 2 of them could mean there is too much digoxin in the blood.

• Feeling dizzy, for which the NHS advises lying down and not driving, riding a bike or using tools or machinery until you feel better.

• Feeling or being sick (nausea or vomiting).

• Diarrhoea, with the risk of dehydration; the NHS says signs include peeing less than usual or dark, strong-smelling pee.

• Changes in vision, including blurred vision and not being able to look at bright light.

• Skin rashes.

• In clinical guidance, visual disturbance with a yellow-green discolouration is described as a toxic symptom that may prompt an urgent blood measurement.

The NHS says to call your doctor or NHS 111 now if you have more than 2 of the common side effects. 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: ask someone to drive you or call 999 for an ambulance, and bring all your medicines.

If you have taken more than your prescribed dose, the NHS says to contact NHS 111 for advice now, and to call 111 if asking about a child under 5. If you need to go to A&E, bring the digoxin packet or leaflet, any remaining medicine, and any other medicines you take.

The BHF says that if you experience heart rhythm disturbances you must go to A&E for an ECG and a blood test, because these are serious and require treatment as soon as possible.

The NHS says children, older people, and people with kidney disease or an underactive thyroid are more sensitive to digoxin, and that doses are usually lower for older people and for people with kidney disease.

The BHF notes that digoxin is cleared from the body by the kidneys, so people with kidney problems may need testing. It also says that digoxin can interact with some common heart medicines, including verapamil and amiodarone, which is why doctors may choose alternatives or avoid starting it.

Clinical guidance from the NHS Specialist Pharmacy Service lists situations in which a level may be checked: adverse effects suggestive of toxicity such as confusion, nausea, anorexia or disturbance of colour vision, use of interacting medicines such as amiodarone, diltiazem or verapamil, deteriorating kidney function, thyroid disease, advancing age, suspected poor adherence and a change in dose.

The NHS says that if you have taken too much, your doctor may ask for a blood test to see how much digoxin is in your blood and to check how well your kidneys are working, and may arrange an electrocardiogram (ECG) to see the effect on your heartbeat.

NHS SPS guidance says routine monitoring of serum digoxin is not recommended but a level should be considered when toxicity is suspected. The sample is taken no less than 6 hours, ideally 8 to 12 hours, after the last dose. It gives a usual therapeutic range of 0.7 to 2.0 nanograms per mL and explains that levels above 3.0 make toxicity likely, between 1.5 and 3.0 make it possible, and below 1.5 in the absence of low potassium make it unlikely.

It adds that toxicity can occur with a level inside the therapeutic range, so results are always interpreted in the clinical context, and that blood potassium, magnesium, calcium and thyroid function tests are part of the picture.

According to NHS SPS guidance, if toxicity occurs digoxin should be withdrawn, and serious manifestations require urgent specialist management. Digoxin-specific antibody fragments are available for reversal of life-threatening overdosage.

Hypokalaemia (low potassium) and hypomagnesaemia (low magnesium) should be corrected either orally or intravenously as appropriate, because they predispose the patient to toxicity. Initial and maintenance doses should be amended when thyroid function is abnormal.

You should not change your own dose. The NHS says that stopping digoxin suddenly can make your condition worse, and that you should talk to your doctor if you want to stop it.

Take it at the same time each day, as the NHS advises, and measure liquid doses using the syringe supplied, not a kitchen spoon. If you miss a dose, the NHS says to leave it out and take the next at the usual time, and not to take two doses.

Keep an up-to-date list of all medicines and tell prescribers and pharmacists that you take digoxin. Attend blood test appointments, and report vomiting, diarrhoea or poor drinking, because dehydration can affect kidney function and mineral levels.

The BHF suggests that if you take digoxin and are worried about your health, you should speak to your doctor and make sure you should be on it, that the dose is correct and that you are being monitored adequately.

Digoxin is prescribed and monitored by the GP or cardiology team on the NHS. Suspected toxicity is an urgent matter handled by NHS 111, GP out-of-hours or A&E, with admission, ECG monitoring and blood tests as needed.

Private cardiology consultations may review the need for digoxin, but any suspected toxicity needs urgent assessment in the same way, and test results and medication changes should be shared with the GP.