Potassium drops when your body loses too much of it through the kidneys or gut, when too little comes in through diet, or when potassium shifts from your bloodstream into your cells. Normal blood potassium ranges from 3.5 to 5.2 mEq/L. A level between 3.0 and 3.5 is considered mild low potassium (hypokalemia), while anything below 3.0 is severe and can affect your heart rhythm and muscle function.
Most cases trace back to one of a handful of common triggers, and understanding which one is at play matters because the fix depends entirely on the cause.
Diuretics Are the Most Common Cause
Water pills prescribed for high blood pressure or fluid retention are the single most common reason potassium drops. These medications work by making your kidneys flush out extra sodium and water, but potassium gets swept out along with them. The loss is dose-dependent: the higher the dose, the more potassium you lose.
Thiazide-type diuretics actually cause low potassium more frequently than loop diuretics, even though loop diuretics are more powerful at removing sodium. If you’re on either type and notice muscle cramps, unusual fatigue, or an irregular heartbeat, your potassium level is worth checking. Some people on these medications take a potassium supplement or a potassium-sparing diuretic alongside them to offset the loss.
Vomiting and Diarrhea
Gastrointestinal losses are another major culprit. Prolonged diarrhea flushes potassium directly out of the body, since about 10% of daily potassium excretion normally happens through the gut. When stool volume increases dramatically, so does potassium loss.
Vomiting works differently than you might expect. Stomach fluid itself contains very little potassium, only about 10 mEq per liter. The real problem is indirect: vomiting causes the body to lose acid, which shifts blood chemistry toward alkalosis. That shift triggers the kidneys to dump potassium in an attempt to restore balance. So the potassium loss from vomiting is mostly happening at the kidneys, not the stomach. Laxative overuse causes the same kind of gut-driven potassium drain, and it’s a particularly common cause in people who use laxatives frequently for weight control.
Too Much Aldosterone
Your adrenal glands produce a hormone called aldosterone that tells your kidneys to hold onto sodium and release potassium. When aldosterone levels run too high, a condition called hyperaldosteronism, the kidneys excrete far more potassium than they should. This can happen because of a small benign growth on one of the adrenal glands or because both glands are overactive.
Hyperaldosteronism is one of the more underdiagnosed causes of persistent low potassium. If your levels keep dropping despite supplements, or if you also have high blood pressure that’s hard to control, excess aldosterone is worth investigating.
Low Magnesium Makes It Worse
Magnesium and potassium are tightly linked. When magnesium is low, the body literally cannot hold onto potassium. Here’s why: magnesium normally acts as a plug on a potassium channel in the kidneys. When intracellular magnesium is adequate, it blocks that channel and keeps potassium from leaking out into the urine. When magnesium drops, the channel opens and potassium pours through.
This is clinically important because if you’re low in both minerals, potassium replacement alone won’t work. The potassium will keep leaking out until magnesium is corrected first. Magnesium deficiency is common in people who drink heavily, take certain medications, or have poor dietary intake, and it’s often overlooked as the hidden reason potassium won’t stay up.
Shifts Between Blood and Cells
Sometimes your total body potassium is fine, but the potassium temporarily moves from your bloodstream into your cells, making blood levels look low. Several things trigger this shift. Insulin pushes potassium into cells, which is why blood sugar spikes (and the insulin response that follows) can cause a temporary drop. Stress hormones like adrenaline do the same, which is why potassium can dip during acute illness, panic attacks, or after heavy exercise.
Alkalosis, a state where your blood becomes too basic (high pH), also drives potassium into cells. This creates a feedback loop: low potassium promotes alkalosis, and alkalosis makes potassium drop further. The relationship runs in both directions, which is why the two problems often show up together.
A rare inherited condition called familial periodic paralysis causes dramatic episodes where potassium suddenly shifts into cells, leading to temporary but sometimes profound muscle weakness or paralysis.
Not Enough Potassium Coming In
Pure dietary deficiency is less common than excess loss, but it happens. Your body takes in roughly 60 to 100 mEq of potassium daily through food, and the kidneys are normally good at conserving it when intake is low. But if you’re eating very little overall, following a highly restrictive diet, or relying on processed foods with minimal fruit and vegetable content, intake can fall short. The risk compounds when poor intake overlaps with another cause of loss, like a diuretic or chronic diarrhea.
Genetic Kidney Disorders
Some people have chronically low potassium because of an inherited defect in how their kidneys handle salt. Gitelman syndrome is one such condition, caused by mutations in a gene that controls salt reabsorption in the kidneys. When this system malfunctions, the kidneys waste not just sodium but also potassium, magnesium, and calcium.
People with Gitelman syndrome often experience muscle cramps, weakness, dizziness, tingling in the face, fatigue, and intense salt cravings. The condition is inherited in a recessive pattern, meaning you need a defective copy of the gene from both parents. A related condition, Bartter syndrome, causes similar electrolyte wasting but typically presents earlier in life and can be more severe. Both are rare but worth considering in someone with persistently low potassium that doesn’t have an obvious explanation.
How Low Potassium Feels
Mild drops (3.0 to 3.5 mEq/L) rarely cause noticeable symptoms. You might feel a bit more tired than usual or have occasional muscle twitches, but many people feel nothing at all.
Moderate drops (2.5 to 3.0 mEq/L) are where things get more serious. Muscle weakness becomes apparent, and in severe cases it can progress to affect breathing muscles. Constipation is common at this stage because the smooth muscle in your gut slows down.
Severe drops (below 2.5 mEq/L) are dangerous. Symptoms include significant muscle cramping, muscle breakdown, low blood pressure, and abnormal heart rhythms. On an ECG, severely low potassium produces characteristic changes including flattened T-waves and prominent U-waves. These heart rhythm disturbances are the main reason severe hypokalemia is treated as a medical emergency. A prolonged QT interval, one of the measurable changes on an ECG, raises the risk of a potentially fatal arrhythmia.
Multiple Causes Often Overlap
In practice, potassium rarely drops for a single reason. A person on a diuretic who develops a stomach bug and stops eating for two days is being hit from three directions at once: medication-driven kidney loss, diarrhea-related gut loss, and reduced intake. Older adults are especially vulnerable to these overlapping scenarios because they’re more likely to be on diuretics, eat less overall, and take longer to recover from illness.
Alcohol use is another common multiplier. Heavy drinking promotes both magnesium loss and poor nutrition, creating the conditions for persistent potassium depletion that resists simple supplementation. Identifying and addressing every contributing factor, not just the most obvious one, is what it takes to get potassium levels back to normal and keep them there.