Alendronate Sodium
(4-amino-1-hydroxy-1-phosphono-butyl) phosphonic acid
Also known as: Fosamax · Adronat · Alendros · Arendal · Onclast · Alendronic acid · Acide Alendronique · Acido Alendronico · Acidum Alendronicum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Refractory hypercalcemia | 0.5-1 mg/kg | PO | once daily | — | 🌎 NA |
| Investigational treatment of histiocytic sarcoma complex | Loading dose of 70 mg/m 2 (NOT mg/kg) | PO | daily for 14 days, then every other day for 14 days, then once weekly | — | 🌎 NA |
- Investigational treatment of histiocytic sarcoma complex: Monitor for esophageal problems (excessive salivation, regurgitation).
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Feline odontoclastic resorptive lesions (FORLs) | 3 mg/kg | PO | q12h | — | 🌎 NA |
| Idiopathic hypercalcemia (after dietary change has been attempted) | Initially 2 mg/kg. Most cats respond to 10 mg (total dose). | PO | once weekly | — | 🌎 NA |
- Feline odontoclastic resorptive lesions (FORLs): Note: Use for this indication in cats is at present very controversial. (Plumb 2006)
- Idiopathic hypercalcemia (after dietary change has been attempted): Administer at least 6 mL of water after administration and butter the lips to increase salivation and increase transit. If efficacious, effects usually seen in 3-4 weeks. Monitor via serum ionized calcium.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Alendronate sodium is an orally administered nitrogen-containing bisphosphonate primarily used to inhibit osteoclastic bone resorption.
Key Clinical Highlights:
- Primary Indications: Utilized in veterinary medicine for refractory hypercalcemia (e.g., idiopathic hypercalcemia in cats, hypercalcemia of malignancy), feline odontoclastic resorptive lesions (FORLs), and as an adjunctive treatment for osteosarcoma or histiocytic sarcoma.
- Investigational Status: Clinical experience in small animals remains highly limited. Dosages and safety profiles are still being established.
- Bioavailability Challenges: Oral absorption is exceptionally poor (<2%) and is drastically reduced by food, requiring strict fasting protocols.
- Esophageal Risk: Known to cause severe esophageal irritation and erosions in humans; specific administration techniques (e.g., water chasers, buttering lips) are required in pets to ensure rapid gastric transit.
Mechanism of action
Alendronate is a nitrogenous bisphosphonate that strongly binds to hydroxyapatite crystals in the bone matrix.
- Mechanism: During bone resorption, the drug is locally released and endocytosed by osteoclasts.
- Intracellular Pathway: Inside the osteoclast, alendronate inhibits the enzyme farnesyl pyrophosphate synthase (FPPS) within the mevalonate pathway → prevents the prenylation (lipid modification) of small GTP-binding proteins (such as Ras, Rho, and Rab) → disrupts osteoclast cytoskeletal organization and ruffled border formation → induces osteoclast apoptosis.
- Secondary Effects: By inhibiting osteoclastogenesis and promoting apoptosis, it indirectly reduces angiogenesis and may inhibit cancer cell proliferation in osteogenic neoplasms.
Safety & warnings
Contraindications
- Esophageal abnormalities (e.g., strictures, achalasia) that delay esophageal emptying
- Inability to stand or sit upright for 30 minutes post-administration (human guideline; veterinary relevance is managed via water chasers/activity)
- Severe renal dysfunction (Creatinine Clearance < 35 mL/min)
- Known hypersensitivity to alendronate or other bisphosphonates
Adverse effects
- Upper GI irritation and erosions
- Esophageal stricture or ulceration
- Vomiting
- Inappetence
- Jaw osteonecrosis (rare but documented in humans)
- Musculoskeletal pain
- Hypocalcemia (with overdose)
- Hypophosphatemia (with overdose)
Precautions
Esophageal Toxicity Warning: Alendronate can cause severe local irritation to the upper gastrointestinal mucosa.
- Administration Technique: To minimize esophageal transit time and prevent erosions, ensure the patient swallows the medication completely. In dogs, walking or playing for 30 minutes post-dose is recommended. In cats, administering at least 6 mL of water and buttering the lips to induce salivation is advised.
- Renal Impairment: Use with extreme caution or avoid in patients with severe renal dysfunction.
- Investigational Use: Inform owners that the use of this drug in veterinary medicine is largely investigational, and long-term safety data is lacking.
Drug interactions
Increased risk of upper GI adverse effects and erosions.
Likely to significantly decrease the already low oral bioavailability of alendronate.
Increased oral alendronate bioavailability two-fold in a human study.
Humans taking NSAIDs with alendronate had no higher rates of GI adverse reactions than when NSAIDs were used with placebo, though caution is still advised.
Significantly decreases alendronate absorption
Decreases alendronate absorption
Monitoring
- Serum ionized calcium
- Serum phosphorus
- Serum potassium and sodium
- Signs of GI adverse effects (excessive salivation, regurgitation, dysphagia, inappetence)
Pharmacokinetics
Half-life
Absorption
Oral bioavailability is extremely low (<2% in all studied species; <1% in humans). The presence of food, coffee, or juice can reduce bioavailability further to negligible amounts.
Distribution
Absorbed drug is rapidly distributed to bone or excreted into the urine. Accumulates on subgingival tooth surfaces and bordering alveolar bone. Not highly plasma protein bound in dogs (but is in rats). Plasma concentrations are virtually undetectable after therapeutic dosing.
Metabolism
Alendronate is not metabolized.
Elimination
Drug taken up by bone is very slowly eliminated. Once incorporated into bone, alendronate is no longer active.
Overdose
Toxicity Profile: No lethality was observed in dogs receiving doses up to 200 mg/kg. Lethality in rodents occurs at >550 mg/kg.
Clinical Signs of Overdose: Hypocalcemia, hypophosphatemia, and severe upper GI reactions (ulceration, erosions).
Treatment:
- DO NOT induce vomiting due to the risk of severe esophageal irritation.
- Administer oral antacids or milk immediately to bind the drug and prevent systemic absorption.
- Monitor serum calcium and phosphorus levels closely.
- Provide supportive care as needed.
Available products
Formulations
- Oral tablets
- Oral solution
Human-labeled
- Alendronate Sodium Oral Tablets: 5 mg, 10 mg, 35 mg, 40 mg, & 70 mg (Fosamax, generic)
- Alendronate Oral Solution: 70 mg (as base) in 75 mL; raspberry flavor (Fosamax)
Regulatory status
POM (Prescription Only Medicine)
POM (Prescription Only Medicine)
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Alendronate tablets should be stored in well-closed containers at room temperature. The oral solution should be stored at room temperature; do not freeze.
Client information
Important Administration Instructions for Pet Owners:
- Strict Fasting: This medication will not be absorbed if given with food. It must be given on an empty stomach, and you must withhold food for at least 30 minutes after dosing.
- Preventing Esophageal Damage: Alendronate can cause severe ulcers in the esophagus if it gets stuck.
- For Dogs: Walk or play with your dog for 30 minutes after giving the pill to keep them upright and ensure the pill reaches the stomach.
- For Cats: Give the pill followed immediately by at least 6 mL (about a teaspoon) of water via syringe. Buttering your cat's lips afterward encourages swallowing and salivation, helping the pill go down safely.
- Watch for Side Effects: Contact your veterinarian immediately if your pet shows signs of drooling, regurgitation, difficulty swallowing, vomiting, or loss of appetite.
- Investigational Use: Understand that the use of this drug in veterinary medicine is relatively new, and long-term effects in pets are still being studied.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.