Phosphorus Binders

If your cat’s phosphorus level is over 6 mg/dl (or 1.9 mmol/L if you’re outside the US), it needs to come down. High phosphorus makes cats feel worse, speeds up the progression of kidney disease, and sets off a cascade that leads to a painful complication called secondary hyperparathyroidism. Diet is the first line of defense, but when diet alone isn’t enough, phosphorus binders are the next step.

This post covers what binders are, the different types available, how to choose between them, and how to dose and give them. If you haven’t already, it’s worth reading up on phosphorus control through diet first, since binders are meant to work alongside a low phosphorus diet, not replace it.

Healthy kidneys regulate phosphorus by filtering out the excess. Once a cat has CKD, the kidneys can no longer do this efficiently, and phosphorus builds up in the blood. This is called hyperphosphataemia, and it’s one of the most consistently important things to manage in a CKD cat. Unlike some other CKD debates where the evidence is mixed, there’s no real dispute here: uncontrolled phosphorus makes cats feel unwell, may speed up kidney damage, and contributes to secondary hyperparathyroidism, a condition covered in more detail in our calcium and PTH post.

A therapeutic kidney diet is the single most effective way to control phosphorus, because it’s formulated to keep phosphorus below 0.5% on a dry matter basis, a level no other complete food matches. If your cat eats a therapeutic kidney diet, you likely won’t need a binder, at least in the earlier stages. Binders come into play when:

• Your cat won’t eat a therapeutic kidney diet
• Your cat’s phosphorus is still too high after a month on the diet
• The disease has progressed to the point where diet alone can’t keep up

If you’re feeding a non-therapeutic food because that’s what your cat will actually eat, a binder becomes even more important, since you’re not getting the phosphorus restriction benefit from the food itself.

Binders work in the gut, not the bloodstream. They attach to phosphorus in food as it moves through the intestine, so that phosphorus is excreted in the stool instead of being absorbed. This means binders must be given with or very close to a meal to do their job. Given on an empty stomach, they’re not doing much.

You should start to see blood phosphorus levels fall about seven to ten days after starting a binder. Binders are not as effective as diet alone, since they can’t bind all the phosphorus in a meal, so even on a binder, feeding the lowest phosphorus food your cat will reliably eat is still worthwhile.

Aluminum Hydroxide

This is the traditional choice and still the one most experienced CKD caregivers reach for first. It’s effective, has no taste or smell in its pure powder or gel form, and is inexpensive. The catch is that many people, including some vets, only know the peppermint-flavoured version most cats hate, which gives aluminum hydroxide an undeserved reputation for being hard to give.

Look for odourless, tasteless loose powder rather than flavoured liquid. Avoid buying from auction sites, since you need pharmaceutical grade material.

There’s long been concern about aluminum toxicity, mostly drawn from human dialysis patients. In cats, this appears to be rare, and most documented cases involved doses far higher than what’s typically used for CKD management. Watch for muscle weakness or an unsteady gait as early warning signs, and note that a falling MCV (red blood cell size) on bloodwork can sometimes appear before physical symptoms.

Constipation is the most common side effect.

Lanthanum Carbonate

About as effective as aluminum hydroxide and similarly tasteless, lanthanum carbonate is a newer option. It’s available under various names depending on your country. The main drawback is cost, since the older feline-specific version was discontinued and current products, including the human medication Fosrenol, are expensive. Side effects can include nausea, vomiting, and constipation, and there have been reports of more serious gastrointestinal obstruction in rare cases, particularly in patients also taking a calcium channel blocker like amlodipine (commonly used for blood pressure in CKD cats).

Calcium-Based Binders

Calcium carbonate (like Tums) and calcium acetate are widely available over the counter and cheap. They don’t bind phosphorus as effectively as aluminum hydroxide or lanthanum, and they carry a real risk of pushing calcium levels too high, so monitoring calcium is important if you go this route. There are dozens of commercial cat-specific products built around calcium carbonate, most with extra ingredients like chitosan added, but they’re considerably more expensive than plain calcium carbonate with no proven added benefit.

Sevelamer

Sevelamer hydrochloride and sevelamer carbonate are used in human dialysis patients and occasionally used in cats, though there’s little research specific to cats. They’re expensive and may cause constipation or interfere with blood clotting, so vitamin K may be needed alongside them.

Iron-Based Binders

A newer category, with products like Lenziaren showing mixed results depending on dose, sometimes even raising phosphorus in healthy cats at higher doses while lowering it in CKD cats. Iron levels need monitoring if you use one of these.

Niacinamide (Vitamin B3)

Niacinamide works differently from the others. Rather than binding phosphorus in food, it blocks the pathways that allow phosphorus absorption in the gut and kidneys, so it doesn’t need to be given with every meal the way true binders do. It’s cheap and widely available, but there’s very little research on its use in cats specifically, and doses need to be increased gradually to avoid a “niacin flush,” a temporary reaction more studied in humans but possible in cats too.

Dosing depends on your cat’s current phosphorus level, weight, and how advanced the CKD is. The International Renal Interest Society’s general guidance for aluminum hydroxide, calcium acetate, calcium carbonate, and lanthanum is a starting range of roughly 30 to 60 mg per kg of cat per day (about 14 to 28 mg per lb), divided across meals. It’s generally best to start at the lower end and increase only if phosphorus doesn’t come down enough, working with your vet throughout.

If your cat needs a fairly large amount of one binder, it can sometimes be better to combine a smaller dose of two different types (for example, aluminum hydroxide plus lanthanum) rather than pushing one binder to a high dose. If you do this, give them at separate times of day since some binders can interact with each other.

Binders need to be mixed into or given right alongside a meal. Tablets can be crushed, capsules opened and sprinkled, and liquid forms mixed in or syringed. For dry food, some people put the kibble and powdered binder in a sealed bag and let them sit together overnight so the powder coats the food.

A few practical notes:

• Give binders one to three hours apart from antibiotics, ACE inhibitors, and stomach acid reducers like famotidine, since binders can interfere with how well these medications are absorbed
• Aluminum hydroxide should be kept away from potassium citrate (found in many therapeutic kidney diets and sometimes given separately) since citrate may increase aluminum absorption
• Multicat households should try to keep binder-dosed food away from healthy cats, especially kittens

Expect to see phosphorus levels start to fall seven to ten days after starting a binder, with your vet rechecking bloodwork around then to see if the dose needs adjusting. Once phosphorus is under control, checks can space out to every two to three months. It’s worth knowing that overshooting is possible too. Low phosphorus is uncommon but not impossible on binders, and it can cause its own problems like weakness, so this is one more reason ongoing monitoring matters rather than just starting a binder and leaving it alone indefinitely.

How do I know if my cat needs a phosphorus binder? Your vet will look at a blood phosphorus level. Generally, a level over 6 mg/dl (1.9 mmol/L) is considered too high for a CKD cat and warrants action, starting with diet and moving to a binder if diet alone (given a month’s trial) isn’t bringing it down enough.

Can I use a binder if my cat is already eating a therapeutic kidney diet? Yes. Many cats eventually need a binder in addition to their kidney diet as CKD progresses, since diet alone becomes less able to keep phosphorus controlled in later stages.

Do phosphorus binders have to be given with every meal? Most do, since they only work by binding phosphorus that’s present in food at the same time. Niacinamide is the exception, since it works through a different mechanism and doesn’t need to be given with every meal.

What’s the safest binder to start with? There’s no single answer that fits every cat. Aluminum hydroxide is usually the first choice for effectiveness and cost, but the right binder depends on your cat’s calcium levels, other medications, budget, and how well your cat tolerates it. This is a decision to make with your vet.

Can binders cause constipation? Yes, this is one of the more common side effects across several binder types, including aluminum hydroxide, lanthanum, and sevelamer. If your cat becomes constipated after starting a binder, talk to your vet about whether a dose adjustment or a change in binder type is needed.

How long will my cat need to stay on a binder? Typically for life, or for as long as the CKD requires phosphorus support. Dosing may need to be adjusted up or down over time as the disease progresses or as bloodwork changes.

• International Renal Interest Society. “IRIS Treatment Recommendations for CKD in Cats.” 2023.
• Elliott J, Elliott D. “Nutritional Management of Feline Chronic Kidney Disease.” Veterinary Focus 18(2), 2008, pp. 39-44.
• Elliott J, Rawlings JM, Markwell PJ, Barber PJ. “Survival of Cats with Naturally Occurring Chronic Renal Failure: Effect of Dietary Management.” Journal of Small Animal Practice 41(6), 2000, pp. 235-242.
• Kidder AC, Chew D. “Treatment Options for Hyperphosphataemia in Feline CKD: What’s Out There?” Journal of Feline Medicine and Surgery 11(11), 2009, pp. 813-824.
• Chew D. “Chronic Kidney Disease (CKD) in Dogs and Cats: Staging and Management Strategies.” Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference.
• Markovich JE, Freeman LM, Labato MA, Heinze CR. “Survey of Dietary and Medication Practices of Owners of Cats with Chronic Kidney Disease.” Journal of Feline Medicine and Surgery 22, 2014.
• Mudge DW, Johnson DW, Hawley CM, Campbell SB, Isbel NM, van Eps CL, Petrie JJB. “Do Aluminum Hydroxide Binders Continue to Have a Role in Contemporary Nephrology Practice?” Nephrology 12, 2011.
• Pepper R, Campbell N, Yaqoob MM, Roberts NB, Fan SLS. “Do Oral Aluminum Phosphate Binders Cause Accumulation of Aluminum to Toxic Levels?” BMC Nephrology 12, 2011.
• Segev G, Bandt C, Francey T, Cowgill LD. “Aluminum Toxicity Following Administration of Aluminum-Based Phosphate Binders in 2 Dogs with Renal Failure.” Journal of Veterinary Internal Medicine 22(6), 2008, pp. 1432-1435.
• Damment SJP. “Pharmacology of the Phosphate Binder, Lanthanum Carbonate.” Renal Failure 33(2), 2011, pp. 217-224.
• Shigematsu T. “Multicenter Prospective Randomized, Double-Blind Comparative Study Between Lanthanum Carbonate and Calcium Carbonate as Phosphate Binders in Japanese Hemodialysis Patients with Hyperphosphatemia.” Clinical Nephrology 70(5), 2008, pp. 404-410.
• Ix JH, Isakova T, Larive B, et al. “Effects of Nicotinamide and Lanthanum Carbonate on Serum Phosphate and Fibroblast Growth Factor-23 in CKD: The COMBINE Trial.” Journal of the American Society of Nephrology 30(6), 2019, pp. 1096-1108.
• Block GA, Wheeler DC, Persky MS, et al. “Effects of Phosphorus Binders in Moderate CKD.” Journal of the American Society of Nephrology 23(8), 2012, pp. 1407-1415.
• King JN, Erasmus HL, Delport PC, Bester ICJ, Seewald W. “Efficacy, Acceptability and Tolerability of the New Oral Phosphate Binder Lenziaren in Healthy Cats Fed a Standard Diet.” BMC Veterinary Research 10(258), 2014.
• King JN, Delport PC, Luus HG, Erasmus HL, Barnes PM, Speranza C. “Efficacy and Acceptability of the New Oral Phosphate Binder Lenziaren in Healthy Cats Fed a Renal Diet.” Journal of Veterinary Pharmacology and Therapeutics 38(3), 2015, pp. 278-289.
• Panel on Additives and Products or Substances Used in Animal Feed. “Scientific Opinion on the Safety and Efficacy of Lenziaren (Iron, Aqua Carbonate Hydroxyl Oxo Starch Sucrose Complex) as a Feed Additive for Cats.” European Food Safety Authority Journal 11(5), 2013, p. 3204.
• Pai AB, Shepler BM. “Comparison of Sevelamer Hydrochloride and Sevelamer Carbonate: Risk of Metabolic Acidosis and Clinical Implications.” Pharmacotherapy 29(5), 2009, pp. 554-561.
• Cheng SC, Young DO, Huang Y, Delmez JA, Coyne DW. “A Randomized Double-Blind Placebo-Controlled Trial of Niacinamide for Reduction of Phosphorus in Hemodialysis Patients.” Clinical Journal of the American Society of Nephrology 3(4), 2008, pp. 1131-1138.
• Vasantha J, Soundararajan P, Vanitharani N, et al. “Safety and Efficacy of Nicotinamide in the Management of Hyperphosphatemia in Patients on Hemodialysis.” Indian Journal of Nephrology 21(4), 2011, pp. 245-249.
• Zahed NS, Zamanifar N, Nikbakht H. “Effect of Low Dose Nicotinic Acid on Hyperphosphatemia in Patients with End Stage Renal Disease.” Indian Journal of Nephrology 26(4), 2016, pp. 239-243.