Low potassium: symptoms, causes and what to do

A low potassium result rarely comes from what you eat. More often diuretics, vomiting, diarrhoea or laxatives are behind it, because they make you lose potassium. It can cause cramps, weakness and palpitations, and it should always be seen by a doctor.

What low potassium means

Potassium keeps your nerves and muscles working, heart included. If you lose more than you take in, the level in your blood drops.

It is usually lost through the urine, for example with diuretics, or through the gut, with vomiting or diarrhoea. It rarely comes from diet alone.

What causes low potassium

The first two are the most common.

• Diuretics: Medicines that make you pass more urine, used for blood pressure and swelling, make you lose potassium. Do not stop them on your own: talk to your doctor.
• Vomiting or diarrhoea: You lose potassium through the gut, especially if it goes on for days.
• Laxatives: Used often or for a long time, they make you lose potassium.
• Liquorice: Eating it often, in sweets or herbal teas too, makes you lose potassium and can raise your blood pressure.
• Some hormones: Too much aldosterone, a hormone made by the adrenal glands, or cortisone taken for a long time. Too much aldosterone usually raises blood pressure too.
• Low magnesium: When magnesium is short, your kidneys lose more potassium, and the level struggles to come back up.
• Heavy sweating: Long efforts in the heat make you lose a little potassium in sweat.

Symptoms

If the drop is mild, often none. When it is more marked, you may notice:

• Muscle weakness and tiredness
• Cramps
• Constipation
• Palpitations or an irregular heartbeat

These symptoms can have other causes too: a blood test helps show whether potassium is involved. With a low potassium, palpitations, very weak muscles or fainting are an emergency: call 112 or go to the emergency department.

Low potassium and your heart

Potassium helps your heart keep its rhythm. When it is low, your heart can make extra or irregular beats, which feel like palpitations.

The risk is higher if the level is very low, if you have heart disease or if you take some heart medicines. That is why your doctor sometimes asks for an ECG, a quick test of your heartbeat.

Which tests to read alongside it

These tests help find the cause and how to correct it.

What your result means

The lower end of the scale, in mmol/L.

• Below 3: Very low. It can cause weakness, cramps and an irregular heartbeat. See your doctor today, or the out-of-hours doctor if you cannot reach them. With palpitations, very weak muscles or fainting, call 112 or go to the emergency department.
• 3 to under 3.5: Low. Diuretics, vomiting or diarrhoea are often behind it. Talk to your doctor soon and do not take potassium supplements on your own. With palpitations, very weak muscles or fainting, call 112 or go to the emergency department.
• Within your report’s limits, Usually 3.5 to 5.1: Normal. Your heart and muscles have the potassium they need. In the Rosa report you also see whether you are in your optimal range.

What to do if your potassium is low

First find the cause, with your doctor: if it stays, potassium drops again. Where to start:

• Below 3.5, talk to your doctor soon. Below 3, see a doctor today.
• What to eat: food alone is usually not enough. Eat more fruit, vegetables, legumes and potatoes only if your doctor suggests it, after checking your kidneys and medicines.
• Potassium supplements only if your doctor prescribes them, and your doctor sets the dose and length after checking your kidneys and medicines.
• If you take diuretics or laxatives, tell your doctor. Do not stop them on your own.
• If you often eat liquorice, cut down and tell your doctor.
• With vomiting or diarrhoea that goes on, drink fluids with added salts and see your doctor.

When to worry

Talk to a doctor if:

• Your potassium is below 3.5.
• It is below 3: see your doctor today, or the out-of-hours doctor if you cannot reach them.
• It is low and you take diuretics or heart medicines.
• It is low and you also have high blood pressure: your doctor may look for a hormone excess.
• Your potassium is out of range and you have palpitations, very weak muscles or fainting: call 112 or go to the emergency department.

With Rosa, a potassium below 3.5 mmol/L is always flagged to a doctor, and below 3 urgently.

Frequently asked questions

Low potassium in older people: why does it happen? With age, people more often take diuretics and laxatives, which make you lose potassium. If an older person has weakness, cramps or palpitations, talk to their doctor: a blood test helps.

Is low potassium dangerous? A slight dip usually causes no trouble right away, but it should always be shown to your doctor, who looks for the cause. Very low levels can cause marked weakness and heart rhythm problems: below 3, see a doctor today.

Low potassium: what should I eat? Food alone is usually not enough: the cause needs finding first. If your doctor suggests more potassium in your diet, bananas, kiwis, apricots, potatoes, legumes and spinach are rich in it.

How is low potassium treated? By removing the cause, for example by reviewing a diuretic with your doctor. If needed, your doctor prescribes a potassium supplement, sometimes with magnesium. If it is very low, it is sometimes treated in hospital.

Does low potassium cause cramps? It can, along with weakness and tiredness. But cramps often have other causes, such as effort or heat: a blood test shows whether potassium is involved.

Sources

• Clase and colleagues, potassium balance and how to manage high and low levels: conclusions of a KDIGO Controversies Conference. Kidney International, 2020. PMID 31706619
• KDIGO clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney International, 2024. PMID 38490803
• Don and colleagues, falsely high potassium caused by fist clenching during the blood draw. New England Journal of Medicine, 1990. PMID 2325722
• Huang and Kuo, why magnesium deficiency makes you lose potassium. Journal of the American Society of Nephrology, 2007. PMID 17804670
• Penninkilampi and colleagues, regular liquorice raises blood pressure and lowers potassium: a systematic review and meta-analysis. Journal of Human Hypertension, 2017. PMID 28660884