Quick answer

Ask your pharmacist or doctor before combining. Loop and thiazide diuretics such as furosemide can lower potassium, and low potassium affects up to 21% of hospital patients, mostly because of diuretics and other medicines. Potassium-sparing diuretics such as spironolactone can raise it instead. Only your prescriber can set a potassium dose, from blood tests.^1,2,3,4

Key facts

The furosemide label says potassium supplements or dietary measures may be needed to control or avoid low potassium.³
The hydrochlorothiazide label says low potassium may be avoided or treated with potassium supplementation or more potassium-rich foods.⁵
The spironolactone label says people taking it should be advised to avoid potassium supplements and high-potassium foods, including salt substitutes.⁴
Prescription potassium chloride must not be used with triamterene or amiloride, its label says.⁶
Blood potassium should be checked frequently in the first few months of furosemide treatment.³

Ask your pharmacist or doctor first

Diuretics move potassium in opposite directions. Potassium-sparing diuretics such as spironolactone and amiloride keep potassium in the body and can raise it too high. Loop and thiazide diuretics such as furosemide and hydrochlorothiazide increase the potassium lost in urine and can lower it too far; their labels say potassium supplements or potassium-rich foods may be needed, as directed by the prescriber.^1,2,3,4,5

On the medicine's label

Two kinds of diuretic, two directions

Loop diuretics such as furosemide and bumetanide, and thiazide diuretics such as chlorothiazide, hydrochlorothiazide and metolazone, increase the potassium lost in urine. NIH says they can lead to abnormally low potassium, and experts recommend monitoring it and starting potassium if warranted.^1,2

Potassium-sparing diuretics such as amiloride and spironolactone do the opposite. They reduce the potassium lost in urine and can push it too high, especially in people whose kidneys are not working well.^1,2

Avoid the combination unless your doctor advises it

The spironolactone label says potassium supplements taken with it may lead to severe hyperkalemia, and that patients should be advised to avoid potassium supplements and foods high in potassium, including salt substitutes.^4,7

On the medicine's label

Who sets the potassium dose

There is no standard potassium dose for people on diuretics. The furosemide and hydrochlorothiazide labels leave it to the prescriber, guided by blood tests, and the potassium chloride label says to adjust the dose to each person's blood potassium.^3,5,6

If you have been prescribed potassium, tell your prescriber before adding any supplement, electrolyte drink or salt substitute that contains it. If you take a potassium-sparing diuretic, ask before adding potassium in any form.^2,4

Potassium-rich foods

MedlinePlus tells people on furosemide or hydrochlorothiazide to follow any instructions to eat more potassium-rich foods, such as bananas, prunes, raisins and orange juice. People on spironolactone are told to avoid potassium-containing salt substitutes and to talk with their doctor about how much potassium-rich food to eat.^7,8,9

Signs to report

The furosemide and hydrochlorothiazide labels list warning signs of fluid and electrolyte imbalance, including dry mouth, thirst, weakness, drowsiness, restlessness, muscle pains or cramps, and nausea or vomiting. NIH lists muscle weakness, palpitations and prickling in the hands and feet among the signs of high potassium.^1,3,5

Questions for your pharmacist

Is my diuretic potassium-sparing, or does it make me lose potassium?
Has a potassium dose been set for me, and when is my next blood test?
Do my electrolyte drinks, multivitamin or salt substitute contain potassium?
Should I change how many potassium-rich foods I eat?

Questions people ask

Do all water pills lower potassium?

No. Loop and thiazide diuretics can lower it, but potassium-sparing diuretics such as spironolactone, amiloride and triamterene keep potassium in the body and can raise it.^2,6

Can I just eat more bananas instead?

Follow your prescriber's plan. MedlinePlus advises following any instructions to eat more potassium-rich foods with furosemide or hydrochlorothiazide, and to discuss potassium-rich foods with the doctor when taking spironolactone.^7,8

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.