Potassium

Is potassium safe?

The potassium pills on a pharmacy shelf are about as low-risk as a supplement gets, mostly because they contain so little. In the US they stop at 99 mg per tablet, while adults need 2,600 to 3,400 mg a day. Healthy kidneys adjust how much potassium leaves in urine, so a few extra tablets make no difference.

Potassium is still one of the more dangerous minerals for the wrong person. Too much in the blood (hyperkalaemia) can stop the heart, and the people most at risk are among the most likely to be told to "eat more potassium": those with high blood pressure, heart failure or kidney disease, many of whom take drugs that hold potassium in the body.

Why the 99 mg limit exists

The number comes from an old safety problem. In the 1960s, enteric-coated potassium chloride tablets were found to cause ulcers and narrowing in the small bowel, where a slowly dissolving tablet released concentrated potassium against the gut wall. Some patients needed surgery and some died.

The FDA responded with a rule for potassium drugs, still in force as 21 CFR 201.306, that coated tablets supplying 100 mg or more of potassium must carry prescription labelling and a warning about "obstruction, hemorrhage, and perforation." The NIH Office of Dietary Supplements notes that the FDA has never ruled whether supplements above 99 mg need the same warning, so most makers simply stay at 99 mg. Multivitamins that contain potassium usually have about 80 mg. Doctors still prescribe higher-dose potassium, now usually as wax-matrix or microencapsulated tablets designed to release slowly, or as liquids and powders, together with blood tests. Even these labels warn of gut ulcers, and the prescribing information for Klor-Con extended-release tablets says they must not be used with the diuretics triamterene or amiloride because of the risk of severe hyperkalaemia.

The side effects of ordinary potassium supplements are mostly stomach upset: nausea, vomiting, discomfort and diarrhoea. Taking them with food helps.

The real risk: kidneys and blood pressure drugs

Hyperkalaemia happens when potassium comes in faster than the kidneys can remove it. Early signs are vague (tingling, weakness), and many people have none until the heart rhythm is affected. Severe cases cause paralysis and cardiac arrest.

With healthy kidneys, the Linus Pauling Institute notes that severe hyperkalaemia needs a single dose above about 18 g, far beyond any supplement, and the NIH cites a study in which healthy people took up to 15,600 mg a day for five days without leaving the normal blood range. With kidney disease, or on an ACE inhibitor or potassium-sparing diuretic, even intakes below the adequate intake can cause hyperkalaemia, because the main exit route is blocked. That is also why neither the US National Academies nor EFSA has set an upper limit: the risk depends on the kidneys, not the dose.

Several common medicines reduce how much potassium the kidneys release:

Medicine	Examples
ACE inhibitors	lisinopril, enalapril, ramipril
Angiotensin receptor blockers (ARBs)	losartan, valsartan, candesartan
Potassium-sparing diuretics	spironolactone, amiloride, triamterene
Antibiotics	trimethoprim (including co-trimoxazole)
Painkillers	NSAIDs such as ibuprofen
Others	heparin, digoxin, beta-blockers

On their own these drugs are routinely used and monitored. The danger is stacking: an ACE inhibitor plus spironolactone, early kidney disease, regular ibuprofen and a potassium supplement or salt substitute. Each step looks harmless, and together they can push potassium high enough to stop the heart. This is the reason we score interactions at 50.

Some drugs work the other way. Loop and thiazide diuretics such as furosemide and hydrochlorothiazide, steroids and some asthma inhalers lower potassium, and doctors may prescribe potassium for this. That is a case for a prescription and blood tests, not for self-treating with supplements.

Salt substitutes and powders

Many low-sodium salt substitutes are made largely of potassium chloride, and potassium powders and electrolyte drinks are sold for sport and keto diets. Salt substitutes contain about 440 to 2,800 mg of potassium per teaspoon, according to the NIH, many times a 99 mg tablet, and they are where people at risk most often run into trouble.

For everyone else they can be a good trade. In a 2021 trial in nearly 21,000 older people with high blood pressure or a past stroke in rural China, a salt substitute with 25% potassium chloride cut strokes by about 14% over almost five years. That result does not answer the question for people with kidney disease or on potassium-raising drugs. If you take any of the drugs in the table or have kidney disease, check with your doctor before switching to a salt substitute.

Product quality

Potassium salts are cheap and chemically simple, and independent testing rarely finds contamination or large label errors. Regulation is the bigger quality control: the 99 mg cap keeps over-the-counterOver-the-counterA medicine you can buy without a prescription, in a pharmacy or shop, with directions on the pack for safe use without a doctor.More in the glossary tablets low-dose. Labels list elemental potassium; 1 mEq is about 39 mg.

Who should be careful

• People with chronic kidney disease, at any stage.
• Anyone on an ACE inhibitor, ARB, spironolactone or another drug in the table.
• People with heart failure or diabetes, who often take several of these drugs.
• Older adults, whose kidney function declines with age even without a diagnosis.

All of them should talk to their doctor or pharmacist before taking potassium in any form, including salt substitutes. Magnesium supplements raise similar issues with kidney disease.

How we scored it

Potassium scores 71/100. Quality (85) and dose safety (80) are high because over-the-counterOver-the-counterA medicine you can buy without a prescription, in a pharmacy or shop, with directions on the pack for safe use without a doctor.More in the glossary tablets hold only 99 mg. Interactions (50) are the weakest factor: several of the most prescribed blood pressure drugs raise potassium, and the combination can be fatal. Organ toxicity (68) reflects heart rhythm risk and gut injuries from coated tablets, and regulatory actions (70) the US restriction on higher-dose tablets.