Acetohydroxamic Acid
N-Hydroxyacetamide
Also known as: Lithostat · Uronefrex · AHA · Acetic acid oxime · N-Acetylhydroxylamide · N-Hydroxyacetamide
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 |
|---|---|---|---|---|---|
| Adjunctive therapy of persistent struvite uroliths and persistent urease-producing bacteria after treating with antibiotics and calculolytic diets | 12.5 mg/kg twice daily PO | PO | twice daily | — | 🌎 NA |
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Acetohydroxamic acid (AHA) is a specialized medication primarily used as an adjunctive treatment in dogs for persistent struvite urolithiasis (bladder stones) and chronic urinary tract infections caused by urease-producing bacteria (e.g., Staphylococcus spp., Proteus spp., Klebsiella, Morganella morganii, and Pseudomonas aeruginosa).
- Clinical Pearl: Because of its high side-effect profile, AHA is generally reserved for refractory cases where standard antibiotic therapy and calculolytic diets have failed.
- Warning: It is strictly contraindicated in cats due to severe toxicity.
Mechanism of action
AHA acts as a potent, reversible inhibitor of the bacterial enzyme urease.
- Urease normally hydrolyzes urea → ammonia + carbon dioxide.
- By inhibiting urease, AHA → decreases urinary ammonia, bicarbonate, and carbonate concentrations → prevents the alkalinization of urine (keeps pH lower).
- This acidic environment increases the solubility of struvite (magnesium ammonium phosphate) and carbonate-apatite crystals, and enhances the efficacy of concurrent antimicrobial therapy (e.g., carbenicillin, gentamicin, clindamycin, trimethoprim-sulfa, or chloramphenicol).
Safety & warnings
Contraindications
- Cats (highly toxic)
- Patients with poor renal function (e.g., serum creatinine >2.5 mg/dL)
- Pregnancy (FDA Category X - severe teratogenic effects)
Adverse effects
- Anorexia
- Vomiting
- Mouth and esophageal ulcers
- Hemolytic anemia
- Hyperbilirubinemia
- Bilirubinuria
- CNS disturbances (anxiety, depression, tremulousness)
- Reticulocytosis
- Bone marrow depression
- Phlebitis
- Skin rashes and alopecia
Precautions
WARNING: Do NOT use in cats; AHA is highly toxic to felines.
- Renal Impairment: Contraindicated in patients with poor renal function (serum creatinine > 2.5 mg/dL).
- Pregnancy: Contraindicated during pregnancy (FDA Category X). In pregnant beagles, doses of 25 mg/kg/day caused cardiac, coccygeal, and abdominal wall abnormalities in puppies. Higher doses caused leg deformities and significant encephalopathologies.
- Adverse Effects: High incidence of adverse effects limits its clinical usefulness. Close monitoring is required.
Drug interactions
AHA may chelate iron salts in the gut if given concomitantly, reducing absorption.
AHA may have a synergistic effect with methenamine in inhibiting urine pH increases caused by urease-producing Proteus spp., and may potentiate antibacterial effects.
In humans, concurrent use of AHA with alcohol has resulted in skin rashes.
Monitoring
- CBC (Complete Blood Count)
- Renal function (including urinalysis)
- Hepatic function (specifically bilirubin levels)
Pharmacokinetics
Absorption
Rapidly absorbed after PO administration. Absolute bioavailability in animals is reported to be 50-60%.
Distribution
Well distributed throughout body fluids.
Metabolism
Partially metabolized to acetamide, which is an active metabolite.
Elimination
36-65% of a dose is excreted in the urine unchanged, and 9-14% excreted in the urine as acetamide. The remainder is reportedly excreted as CO2 via the respiratory tract.
Overdose
Acute overdoses can lead to severe hematologic and neurologic signs.
- Clinical Signs: Anorexia, tremors, lethargy, vomiting, and anxiety.
- Hematologic Effects: Increased reticulocyte counts and severe hemolytic reactions. Mild overdoses over several weeks can also result in hemolysis, particularly in patients with reduced renal function.
- Treatment: Intensive hematologic monitoring with adjunctive supportive therapy, including possible blood transfusions.
Available products
Formulations
- Oral tablets
Human-labeled
- Acetohydroxamic Acid Oral Tablets: 250 mg (Lithostat)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Tablets should be stored in tight containers.
Client information
Acetohydroxamic acid is used to help dissolve stubborn bladder stones and treat specific urinary tract infections in dogs.
- Give exactly as prescribed and do not give to pregnant animals or cats.
-
Important: This medication has a high risk of side effects.
- Watch for and report immediately: Persistent or severe vomiting, loss of appetite, changes in urine color (dark or bloody), yellowing of the whites of the eyes (jaundice), or extreme tiredness/decreased energy.
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.