Calcium Acetate (PhosLo)
calcium acetate — Brand names: PhosLo, Phoslyra, Eliphos, Calphron
Indications for Calcium Acetate
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Hyperphosphatemia in end-stage renal disease (ESRD) | Adults on dialysis | Adjunctive to dietary phosphate restriction and dialysis | FDA Approved |
Calcium acetate is approved specifically as a phosphate binder in patients with ESRD who require reduction of serum phosphorus. It functions by chelating dietary phosphate in the gastrointestinal lumen, forming an insoluble calcium phosphate complex that is eliminated in the feces. It is intended to complement dietary phosphorus restriction and dialysis, not to replace them. Calcium acetate does not promote aluminium absorption, which historically was a concern with aluminium-based phosphate binders (FDA PI).
The KDIGO 2017 CKD-MBD guideline update recommends phosphate-lowering agents for patients with progressively or persistently elevated serum phosphorus but does not favour one phosphate binder over another. The guideline does advise restricting the dose of calcium-based phosphate binders in adults with CKD G3a–G5D to limit cumulative calcium load.
Hyperphosphatemia in pre-dialysis CKD (stages 3–5): Used when dietary restriction alone fails. Evidence quality: Moderate. KDIGO advises caution with calcium load in this population.
Hyperphosphatemia in peritoneal dialysis: Applied with same rationale as hemodialysis; clinical trial data are limited to hemodialysis populations. Evidence quality: Low.
Dosing of Calcium Acetate
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| ESRD on hemodialysis — initial phosphate reduction | 1334 mg (2 gelcaps) with each meal | 2001–2668 mg (3–4 gelcaps) with each meal | Individualized; guided by serum Ca and P | Titrate every 2–3 weeks based on serum phosphorus; FDA PI target: serum P <6 mg/dL (KDIGO: toward normal range) Each 667 mg gelcap = 169 mg (8.45 mEq) elemental calcium |
| ESRD on peritoneal dialysis — phosphate control | 1334 mg (2 gelcaps) with each meal | 2001–2668 mg (3–4 gelcaps) with each meal | Individualized | Monitor closely; consider dialysate calcium concentration Lower dialysate Ca may offset hypercalcemia risk |
| Elderly dialysis patient — cautious initiation | 1334 mg (2 gelcaps) with each meal | Titrate slowly per labs | Individualized | Start at low end of range; higher risk of hypercalcemia and cardiac comorbidities FDA PI recommends cautious dosing in elderly |
| CKD stage 4–5 (pre-dialysis) — off-label phosphate lowering | 667–1334 mg with each meal | Guided by serum P and Ca | KDIGO advises restricting calcium-based binder dose | Limit total elemental calcium from binders; monitor Ca x P product KDIGO 2017 recommends restricting calcium-based binder dose |
| Oral solution formulation (Phoslyra) — ESRD | 10 mL (1334 mg) with each meal | 15–20 mL (2001–2668 mg) with each meal | Individualized | 667 mg per 5 mL; use provided dosing cup only Diarrhea more common with solution vs capsules (13.2% vs lower rates) |
Calcium acetate must be taken with meals to be effective. Its phosphate-binding action occurs in the GI tract and depends on the presence of dietary phosphorus. Taking it on an empty stomach provides no phosphate binding benefit and increases the proportion of calcium absorbed systemically (up to 40% fasting vs ~30% with food), raising the risk of hypercalcemia. In the pivotal 12-week study, the average effective dose after titration was 3.4 tablets per meal (FDA PI).
Pharmacology of Calcium Acetate
Mechanism of Action
Calcium acetate is a highly water-soluble calcium salt that dissociates readily at neutral pH in the proximal small intestine. The liberated calcium ions bind to dietary phosphate in the gut lumen, forming insoluble calcium phosphate complexes that resist absorption and are excreted in the feces. This chelation mechanism is the basis for its use as a phosphate binder in ESRD patients. The high solubility of calcium acetate at physiological pH means it releases calcium more rapidly in the upper GI tract than calcium carbonate, which requires acidic conditions for dissolution. This property allows effective phosphate binding even in patients taking proton pump inhibitors, an advantage over carbonate-based binders. In the pivotal cross-over study, two weeks of treatment reduced serum phosphorus by a mean of 19% (p < 0.01) from baseline (FDA PI).
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | ~30% systemic absorption with food; ~40% fasting | Must be dosed with meals for phosphate binding; fasting use increases hypercalcemia risk without therapeutic benefit |
| Distribution | Absorbed Ca2+ distributes into bone (~99%), teeth, and extracellular fluid; ~40% protein-bound in serum | Serum calcium reflects only the small ionized and albumin-bound pool; correct for albumin in hypoalbuminemia |
| Metabolism | Not metabolized; ionic calcium is incorporated into physiological pathways (bone mineralization, signalling) | No hepatic dose adjustment required; no CYP-mediated drug interactions |
| Elimination | Unabsorbed calcium phosphate complex: fecal; absorbed Ca2+: renal filtration and tubular reabsorption | ESRD patients lack the renal escape mechanism for excess calcium, increasing hypercalcemia risk; dialysis can remove some excess |
Side Effects of Calcium Acetate
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Hypercalcemia (serum Ca >11 mg/dL) | 16.3% (3-month open-label study); 7.2% (2-week controlled study) | Most common reason for dose reduction; all cases in pivotal trial resolved on dose decrease or discontinuation |
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Nausea | 6.1% (3-month study); 3.6% overall | More common in the liquid formulation; may be lessened by splitting the dose around the meal |
| Vomiting | 4.1% (3-month study); 2.4% overall | Typically mild; evaluate calcium level if vomiting persists as it may signal early hypercalcemia |
| Diarrhea (oral solution only) | 13.2% (liquid formulation study) | Significantly more frequent with the oral solution than capsules; may be related to maltitol content in some formulations |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Severe hypercalcemia (Ca >12 mg/dL) | Uncommon (~2–5% at high doses) | Weeks 1–4 of dose titration | Discontinue calcium acetate; acute hemodialysis may be required; associated with confusion, delirium, stupor, and coma |
| Vascular and soft-tissue calcification (chronic risk) | Unknown (long-term concern) | Months to years of cumulative calcium loading | Maintain Ca x P product <55 mg²/dL²; periodic radiographic evaluation of at-risk areas; consider non-calcium binder if calcification progresses |
| Digitalis toxicity potentiation | Rare (dose-dependent on both agents) | Any time during concurrent use | Avoid co-prescribing if possible; if required, closely monitor serum calcium and ECG for arrhythmias |
| Hypersensitivity / Pruritus | Rare (isolated case reports) | Variable | Discontinue if allergic reaction suspected; switch to a non-calcium phosphate binder |
| Reason for Discontinuation / Dose Change | Incidence | Context |
|---|---|---|
| Hypercalcemia (most common) | 16.3% | 3-month study; all resolved with dose adjustment; no permanent discontinuations reported specifically |
| GI intolerance (nausea / vomiting) | ~3–6% | Most resolved with continued therapy or dose modification; higher with oral solution |
Mild hypercalcemia (10.5–11.9 mg/dL) is usually managed by reducing the calcium acetate dose or temporarily pausing therapy. Concurrently reduce or stop vitamin D and its analogues, and evaluate the dialysate calcium concentration — lowering dialysate calcium from 2.5 mEq/L to 2.0 mEq/L can significantly mitigate binder-induced hypercalcemia. Severe hypercalcemia (>12 mg/dL) requires emergency management, which may include acute hemodialysis and permanent discontinuation of calcium acetate (FDA PI).
Drug Interactions with Calcium Acetate
Calcium acetate is not metabolized by cytochrome P450 enzymes and does not undergo hepatic processing. Its drug interactions occur almost entirely through direct chelation and binding in the GI tract: the free calcium ions bind to anionic functional groups (carboxyl, hydroxyl) on co-administered drugs, forming insoluble complexes that reduce absorption of the interacting medication. This mechanism affects multiple drug classes commonly used in the ESRD population.
Monitoring for Calcium Acetate
-
Serum Calcium
Twice weekly during titration; then q1–3 months
Routine Critical parameter. Target normal range (8.4–10.2 mg/dL). Correct for albumin. Reduce dose or hold if Ca >10.5 mg/dL. Frequency should increase whenever the dose is adjusted (FDA PI). -
Serum Phosphorus
Every 2–3 weeks during titration; then q1–3 months
Routine Primary efficacy measure. FDA PI target: <6 mg/dL; KDIGO recommends lowering toward normal range (3.5–5.5 mg/dL). Determines dose titration intervals. -
Ca x P Product
At each calcium/phosphorus check
Routine Maintain below 55 mg²/dL². Elevated product increases risk of ectopic calcification in soft tissues and vasculature (FDA PI). -
Intact PTH
Every 3–6 months (CKD G5D)
Routine Persistent suppression of PTH may indicate adynamic bone disease from calcium overload; consider switching to a non-calcium binder if PTH is persistently low (KDIGO 2017). -
Signs of Hypercalcemia
Every visit
Trigger-based Ask about constipation, anorexia, nausea, vomiting, confusion, or lethargy. Severe hypercalcemia (>12 mg/dL) can cause delirium, stupor, and coma. -
Vascular Calcification
Baseline; then as clinically indicated
Trigger-based Lateral abdominal radiograph or CT for vascular calcification scoring (KDIGO 3.3.1). Consider switching to non-calcium binder if calcification progresses during therapy.
Contraindications & Cautions for Calcium Acetate
Absolute Contraindications
- Hypercalcemia (serum calcium >10.5 mg/dL): The only labelled contraindication. Additional calcium loading worsens hypercalcemia and its complications, including cardiac arrhythmia, altered mental status, and ectopic calcification (FDA PI).
Relative Contraindications (Specialist Input Recommended)
- Concurrent digitalis therapy: Hypercalcemia potentiates digitalis toxicity. Patients on cardiac glycosides were excluded from clinical trials. If unavoidable, requires close calcium and ECG monitoring under specialist supervision.
- Known or progressive vascular calcification: KDIGO recommends restricting calcium-based binder dose in patients with established vascular or valvular calcification. A non-calcium binder (sevelamer, lanthanum, sucroferric oxyhydroxide) may be preferable.
- Adynamic bone disease / persistently suppressed PTH: Low PTH states indicate reduced bone buffering capacity for calcium, increasing hypercalcemia risk even at standard doses.
Use with Caution
- Elderly patients: Start at the low end of the dosing range; greater frequency of decreased cardiac, hepatic, and renal function increases susceptibility to hypercalcemia-related adverse events (FDA PI).
- Patients on vitamin D or calcimimetics: Concurrent active vitamin D therapy increases GI calcium absorption, magnifying the risk of hypercalcemia. Adjust vitamin D dose when initiating or up-titrating calcium acetate.
- Pregnancy: Category C; no controlled human studies. Hypercalcemia during pregnancy may increase risk of stillbirth, preterm delivery, and neonatal hypocalcemia. Use only if benefit outweighs risk, with frequent calcium monitoring.
- Concurrent use of other calcium supplements or calcium-based antacids: Avoid co-administration; additive calcium loading increases hypercalcemia risk.
The KDIGO 2017 CKD-MBD guideline update recommends restricting the dose of calcium-based phosphate binders in adults with CKD G3a–G5D. The long-term effect of calcium acetate on the progression of vascular or soft-tissue calcification has not been determined (FDA PI). Observational and randomized data suggest that high cumulative calcium load from binders may contribute to cardiovascular calcification, though a direct causal link remains under investigation. Prescribers should weigh this consideration when selecting a phosphate binder, especially in patients with pre-existing calcification.
Patient Counselling for Calcium Acetate
Purpose of Therapy
Calcium acetate lowers high phosphorus levels in the blood, which is a common problem when the kidneys are no longer able to remove enough phosphorus on their own. High phosphorus can weaken bones, cause itching, and damage blood vessels and the heart over time. This medication works by grabbing onto the phosphorus in food before it can be absorbed into the bloodstream, so the phosphorus leaves the body with stool instead.
How to Take
Take calcium acetate with each meal — this is essential for it to work. If you skip a meal, skip that dose of calcium acetate as well. Swallow capsules whole with water during or immediately after eating. If prescribed the liquid solution (Phoslyra), use only the measuring cup provided; do not use a household spoon. Do not take calcium acetate on an empty stomach, as this provides no phosphorus-lowering benefit and increases the risk of high calcium levels. Follow the phosphorus-restricted diet prescribed by your healthcare team.
Sources
- PhosLo (calcium acetate) 667 mg gelcaps — Full Prescribing Information. Fresenius Medical Care North America. Revised 03/2011. FDA label (PDF) Primary regulatory source for dosing, contraindications, adverse reactions, and drug interactions in this monograph.
- Phoslyra (calcium acetate oral solution) 667 mg/5 mL — Full Prescribing Information. Fresenius Medical Care North America. DailyMed Source for oral solution-specific adverse reactions (notably diarrhea rate of 13.2%) and bioequivalence data with gelcap formulation.
- PhosLo (calcium acetate) tablet formulation — Prescribing Information. DailyMed Older tablet formulation label providing additional clinical pharmacology data and geriatric dosing guidance.
- Calcium acetate pivotal efficacy study: 12-week open-label trial (n=91 evaluable) in ESRD hemodialysis patients demonstrating 30% reduction in serum phosphorus (p<0.01). Reported in FDA PI, Section 14. Primary efficacy evidence for FDA approval; established mean effective dose of 3.4 tablets per meal after titration.
- Calcium acetate double-blind placebo-controlled cross-over study (n=69 ESRD patients). 2-week treatment with calcium acetate reduced serum phosphorus by 19% vs placebo (p<0.01). Reported in FDA PI, Section 14. Controlled efficacy evidence confirming phosphate-binding effect and adverse reaction rates versus placebo.
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD-MBD Work Group. KDIGO 2017 clinical practice guideline update for the diagnosis, evaluation, prevention, and treatment of chronic kidney disease–mineral and bone disorder (CKD-MBD). Kidney Int Suppl. 2017;7(1):1–59. KDIGO PDF Authoritative guideline recommending restriction of calcium-based phosphate binder dose in CKD G3a-G5D; used for monitoring intervals and phosphate targets.
- KDIGO 2024 CKD-MBD Controversies Conference Report. Kidney Int. 2025. KDIGO PDF Most recent KDIGO position reviewing cumulative calcium load, vascular calcification risk, and nuanced approach to binder selection.
- Slatopolsky E, Weerts C, Lopez-Hilker S, et al. Calcium carbonate as a phosphate binder in patients with chronic renal failure undergoing dialysis. N Engl J Med. 1986;315(3):157–161. doi:10.1056/NEJM198607173150304 Foundational study establishing calcium salts as effective phosphate binders in ESRD, providing mechanistic context for calcium acetate’s development.
- Zamfirescu I, Carlson HE. Absorption of levothyroxine when coadministered with various calcium formulations. Thyroid. 2011;21(5):483–486. doi:10.1089/thy.2010.0296 Prospective PK study in 8 healthy adults showing calcium acetate reduces levothyroxine absorption by 20–25%, supporting separation of dosing.
- Natale P, Green SC, Ruospo M, et al. Phosphate binders for preventing and treating chronic kidney disease-mineral and bone disorder (CKD-MBD). Cochrane Database Syst Rev. 2025;6:CD006023. doi:10.1002/14651858.CD006023.pub4 Comprehensive Cochrane review (updated 2025) comparing phosphate binders; calcium-based binders associated with higher hypercalcemia risk versus non-calcium alternatives.
- Wazny LD, Czarnecki A. Calcium acetate versus calcium carbonate as phosphorus binders in patients on chronic hemodialysis. Ann Pharmacother. 1997;31(2):241–245. doi:10.1177/106002809703100218 Comparative pharmacokinetic analysis showing calcium acetate binds approximately twice as much phosphate per gram of elemental calcium versus calcium carbonate.
- Hsu CH, Patel SR, Young EW, Vanholder R. The biological action of calcitriol in renal failure. Kidney Int. 1994;46(3):605–612. doi:10.1038/ki.1994.314 Provides pharmacological context for the interplay between calcium-based phosphate binders and vitamin D metabolism in ESRD.