Sodium polystyrene sulfonate (SPS) is a medication that lowers potassium levels in the blood by swapping sodium for potassium inside the gut. Sold under brand names like Kayexalate and Kionex, it has been the go-to treatment for hyperkalemia (high potassium) for decades. It works slowly, typically taking 2 to 6 hours to start lowering potassium, and its effects can last anywhere from 6 to 24 hours.
How It Works
SPS is a cation exchange resin, which is a fancy way of saying it’s a material that trades one mineral for another. When it reaches the colon, it releases sodium ions and grabs potassium ions in return. The potassium then leaves the body through the stool instead of staying in the bloodstream. The resin isn’t selective, though. It also binds calcium and magnesium, which means those levels can drop as a side effect and need monitoring.
Because the exchange happens primarily in the colon, it can be given either by mouth or as a rectal enema. The oral form is available as a powder that gets mixed with water or syrup, a pre-mixed liquid suspension, or a powder for suspension. When taken by mouth, the resin travels through the entire digestive tract before doing most of its work in the large intestine.
What It’s Prescribed For
SPS is FDA-approved for treating hyperkalemia, a condition where potassium in the blood rises high enough to disrupt heart rhythm and muscle function. Normal blood potassium falls between about 3.5 and 5.0 milliequivalents per liter; levels above that range can become dangerous, especially for people with kidney disease who can’t efficiently clear potassium on their own.
One important limitation: SPS is not a good choice for emergencies. Its variable onset (somewhere between 2 and 6 hours, sometimes longer) means it can take hours to days to meaningfully lower potassium. In situations involving rapid tissue breakdown, such as severe burns or acute kidney failure, or when potassium is critically elevated, faster interventions like dialysis are necessary. SPS is better suited for milder or chronic elevations where there’s time for it to take effect.
Side Effects and Safety Concerns
The most talked-about risk with SPS is intestinal injury. Cases of intestinal necrosis (tissue death in the bowel), bleeding, perforation, and a type of reduced blood flow called ischemic colitis have been reported. The FDA notes that the majority of these serious gut complications occurred in patients who also received sorbitol, a sugar alcohol that was historically mixed with SPS to prevent constipation. The FDA now recommends against combining SPS with sorbitol.
That said, the actual incidence of these events appears low. A study published in The American Journal of Medicine tracked hospitalized patients over nine years and found the cumulative rate of colonic necrosis was 0.14% in patients who received SPS, compared with 0.07% in those who did not. The difference was not statistically significant. Still, certain people face higher risk: those with a history of bowel disease or surgery, dehydration, kidney failure, or premature infants.
More common, everyday side effects include constipation, nausea, and appetite loss. Because the resin dumps sodium into the gut while pulling out potassium, it can raise sodium levels in the blood. People on sodium-restricted diets, including many heart failure and kidney disease patients, need careful monitoring. Calcium and magnesium levels should also be checked, since the resin binds those minerals too. The FDA recommends frequent potassium checks, at minimum every 24 hours, while someone is taking SPS.
How It Compares to Newer Options
For decades, SPS was the only FDA-approved potassium binder. Two newer alternatives have changed the landscape significantly.
Sodium zirconium cyclosilicate (sold as Lokelma) works faster and more predictably. It begins lowering potassium within about 1 hour, and in patients with potassium levels between 6.1 and 7.2 milliequivalents per liter, a single dose brought more than half of them below 5.5 within 4 hours. Evidence-based reviews consider it the preferred agent when potassium needs to come down quickly.
Patiromer (sold as Veltassa) takes longer to kick in, around 7 hours, and produces a more modest initial reduction. It is not recommended for acute situations. However, clinical data support it as the preferred choice for managing chronic hyperkalemia over the long term, particularly in patients who need ongoing potassium control while staying on heart or kidney medications that tend to raise potassium.
SPS remains in widespread use partly because of its long track record, wide availability, and low cost. But its variable onset, sodium load, and lack of large modern efficacy trials put it at a disadvantage compared to these newer options. For patients who can’t access or tolerate the alternatives, SPS still fills an important role, with the understanding that it works best in non-urgent situations where close lab monitoring is in place.