Electrolyte problems during chemotherapy

Electrolytes are salts and minerals dissolved in the blood, including sodium, potassium, magnesium, calcium and phosphate. They keep nerves firing, muscles contracting, the heart beating in rhythm and fluid in the right places. During chemotherapy their levels can drop. Vomiting and diarrhoea wash them out, poor eating reduces intake, and some medicines make the kidneys lose them. Cisplatin is well known for causing low magnesium and potassium, and some targeted antibodies such as cetuximab and panitumumab cause low magnesium. Water tablets, steroids and some antibiotics also play a part.

Low levels are common and usually correctable. Low magnesium and potassium cause muscle cramps, weakness, tiredness, twitching and, when severe, an irregular heartbeat. Low sodium can cause headache, nausea, confusion and, rarely, seizures. Low calcium causes tingling around the mouth and fingers and muscle spasms. Because symptoms are vague and can look like general treatment tiredness, they are often found on routine blood tests before they are noticed. Supplements by mouth or a drip usually bring levels back. This page explains the tests; it cannot interpret your results or tell you what supplements to take.

The kidneys control the balance

Medicines that affect the kidneys can cause salts to be lost in the urine for weeks or longer.

Fluid losses drain salts quickly

A day of heavy vomiting or diarrhoea can lower potassium and sodium significantly.

Replacement is routine

Magnesium and potassium are often added to drips with cisplatin, and tablets may be prescribed at home.

It cannot tell you whether your electrolyte results are a concern. Interpreting them depends on how low they are, how fast they fell, your symptoms, your heart and kidney health, and your medicines. Only your team can judge this.

It also cannot tell you which supplements to take or how much. Too much potassium or magnesium can be dangerous, especially if the kidneys are not working well. Take supplements only as prescribed.

Do not self-treat with salt or supplements

Adding extra salt, electrolyte powders or mineral tablets without advice can upset the balance further.

Oral rehydration solution helps with losses

For mild diarrhoea or vomiting, oral rehydration solution replaces both fluid and salts. Ask your team whether it is suitable for you.

Plain water alone is not always enough

Drinking large amounts of plain water during heavy losses can lower sodium. Mix in oral rehydration solution, soups or buttermilk as advised.

Food sources can support levels

Bananas, coconut water, potatoes, dal, green leafy vegetables, nuts and whole grains contain potassium and magnesium, if your team agrees.

Some medicines interact

Water tablets, heart medicines and acid-reducing tablets can affect salts. Tell your team about all medicines.

Magnesium tablets can cause diarrhoea

If supplements upset your stomach, tell your team rather than stopping them. The form or timing can be changed.

Low levels can continue after treatment

After cisplatin, some people keep losing magnesium for months and need ongoing checks.

Older adults and heart patients

They are more sensitive to low potassium and magnesium, which can affect heart rhythm.

Keep reports together

Trends in salts over several tests help your team plan replacement.

Keep a record of supplements and symptoms

If you are prescribed magnesium or potassium, note when you take them and whether cramps, weakness or palpitations improve. Bring the record to appointments so your team can adjust doses based on both blood tests and how you feel.

Hot weather adds to losses

Sweating in summer and long journeys to treatment increase fluid and salt losses. Drink regularly, rest in shade, and ask whether oral rehydration solution is suitable on hot days.

Symptoms can build slowly

Low magnesium or potassium often causes gradual tiredness, weakness and cramps over several days, which are easy to blame on treatment. If you feel steadily weaker between cycles, ask whether your salts have been checked recently.

Children and older adults

Both can become unwell quickly with salt changes and may not describe symptoms clearly. Watch for unusual sleepiness, confusion or reduced movement.

What to do next

Report cramps, weakness, palpitations, confusion or ongoing vomiting and diarrhoea, keep blood test appointments, take supplements only as prescribed, avoid self-treating with salt or mineral products, and seek urgent help for an irregular heartbeat, severe weakness or confusion.