Potassium supplements: when they are prescribed, how they are taken and the risks

Potassium is a mineral that the NHS says helps control the balance of fluids in the body and helps the heart muscle work properly. The National Kidney Federation adds that it is important for the normal function of all nerves and muscles, including the heart, that the kidneys control how much potassium is in the body, and that a normal blood level is 3.5 to 5.0 mmol per litre.

Most people get enough from food. The NHS says potassium is found in most types of food, including bananas, vegetables such as broccoli, parsnips and Brussels sprouts, beans and pulses, nuts and seeds, fish, beef, chicken and turkey, and that adults aged 19 to 64 need 3,500 mg a day.

Because the amount in the blood is tightly controlled, both too little (hypokalaemia) and too much (hyperkalaemia) can affect heart rhythm and muscles. That is why potassium is a medicine that is taken on advice, not casually.

Patient.info’s professional reference, written by UK doctors, defines hypokalaemia as a serum potassium concentration below 3.5 mmol/L and says that most cases result from diuretic use or loss of gastrointestinal fluids through persistent vomiting, chronic diarrhoea or laxative misuse. Thiazide and loop diuretics are the most common cause. Other causes include low magnesium, some hormonal conditions, kidney conditions and, rarely, an inherited tendency such as Gitelman’s syndrome.

Mild low potassium is usually without symptoms. In more severe cases the article lists lassitude, generalised weakness and muscle pain and constipation, and at very low levels severe muscle weakness, paralysis and breathing muscle involvement. It says all patients with moderate or severe hypokalaemia should have an ECG, because low potassium can affect the heart and cause arrhythmias.

The same source says it is seldom necessary to use potassium supplementation with low-dose diuretics used for blood pressure, but that people on higher-dose diuretics are likely to need preventive measures. Where possible, a potassium-sparing diuretic such as spironolactone or amiloride may be preferable to oral supplements for people on potassium-lowering diuretics.

The National Kidney Federation notes that low potassium can develop in some people taking certain tablets or with poor food intake, while in kidney disease the problem is more often high potassium.

Patient.info describes management of hypokalaemia as almost always potassium replacement, with the amount depending on the severity, and says dietary supplementation and treating the underlying cause are part of treatment. For mild hypokalaemia and low-risk moderate hypokalaemia it describes oral treatment with potassium supplementation of 40 to 120 mmol a day, depending on severity, with regular monitoring from weekly to several times weekly.

Potassium supplements are usually given as potassium chloride in divided doses. Modified-release preparations have been associated with cases of gastric erosions, so the British National Formulary recommends liquid-based or effervescent preparations, named in the article as Kay-Cee-L and Sando-K, rather than modified-release preparations such as Slow-K, as first line. Potassium phosphate can be used when potassium and phosphate are both depleted, and potassium bicarbonate where there is also metabolic acidosis.

Severe hypokalaemia, or hypokalaemia in a high-risk person, should be corrected in hospital as it is potentially hazardous, and where oral replacement is not possible potassium is given intravenously at carefully controlled rates. The article warns never to give potassium chloride as a bolus because it can cause fatal arrhythmias.

Always take any potassium preparation exactly as directed, with the dilution and food advice on the label, and do not buy potassium supplements to treat symptoms such as cramps without a blood test.

The NHS says that taking too much potassium can cause stomach pain, feeling sick and diarrhoea, and that taking 3,700 mg or less of potassium supplements a day is unlikely to have obvious harmful effects. It says that older people may be more at risk because their kidneys may be less able to remove potassium from the blood, and that they should not take potassium supplements unless advised by a doctor.

The National Kidney Federation says high potassium can interfere with normal muscle and nerve function and cause the heart to beat irregularly. In kidney disease, medicines such as ACE inhibitors and other factors such as constipation and blood transfusions can contribute to high levels. It warns not to restrict potassium unless advised and not to use potassium-rich salt substitutes if you have been told to cut down.

Several medicine combinations raise potassium. A page on this site covers the combinations that push potassium up, such as ACE inhibitors, potassium-sparing diuretics, trimethoprim and others. If you are prescribed potassium alongside any of these, expect more frequent blood tests.

Patient.info lists avoiding the combination of a potassium-sparing diuretic and potassium supplements in some situations, which shows why the choice of prevention is individual.

Patient.info says blood tests such as urea and electrolytes, magnesium and bicarbonate help identify the cause, and that low magnesium often accompanies hypokalaemia and needs to be corrected to enable recovery of potassium. Treatment targets in higher-risk patients may be 4.5 mmol/L or more, though care is needed in kidney insufficiency.

If you are on long-term supplements, ask when your next blood test is due and who is responsible for it. Report symptoms of too little or too much potassium, such as new weakness, palpitations, cramps or constipation, and tell the prescriber about any new medicine, including over-the-counter anti-inflammatory painkillers, which can affect kidney function.

Dietary change can be as effective as supplements for prevention in some situations according to the article, so ask about food sources; never use salt substitutes containing potassium chloride without checking.

On the NHS, low potassium is found on routine blood tests and managed by GPs or hospital teams, with supplements prescribed and monitored. Private clinics follow the same guidance. Buying potassium over the counter to self-treat is not advised, because of the risk of harm without a blood test.