Zidovudine
3'-azido-2',3'-dideoxythymidine
Also known as: Retrovir · AZT · ZDV · azidodeoxythymidine · 3'-azido-2',3'-dideoxythymidine · azidothymidine · BW-A509U · BW-509U · compound-S · zidovudinum
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
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive therapy of FeLV and FIV | 5-10 mg/kg PO or SC q12h | PO, SC | q12h | — | 🌎 NA |
| Adjunctive therapy of FeLV and FIV | 5-10 mg PO or SC q12h | PO, SC | q12h | — | 🌎 NA |
| FeLV | 5 mg/kg PO or SC q12h | PO, SC | q12h | — | 🌎 NA |
| FIV encephalopathy | 20 mg/kg PO q12h | PO | q12h | — | 🌎 NA |
- Adjunctive therapy of FeLV and FIV: Higher dose should be used carefully as side effects can develop. For SC injection, dilute lyophilized product in isotonic sodium chloride to prevent local irritation. For PO, syrup or gelatin capsules can be used.
- Adjunctive therapy of FeLV and FIV: Note: Dose is in mg per cat, not mg/kg. Higher dose should be carefully used in FeLV-infected cats because side effects, particularly non-regenerative anemia, can develop.
- FeLV: If giving SC, dilute in sterile normal saline to prevent local irritation. Check CBC weekly the first month.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Zidovudine (AZT) is a nucleoside reverse transcriptase inhibitor (NRTI) used primarily in veterinary medicine as an adjunctive treatment for feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV).
- Clinical Utility: While it can reduce the viral load in infected cats and improve clinical signs (such as stomatitis or neurological manifestations), it is not a cure and may not significantly alter the natural long-term course of the disease.
- Clinical Pearl: Because retroviruses integrate into the host genome, NRTIs like zidovudine are most effective at preventing the infection of new cells rather than clearing already infected cells.
Mechanism of action
Zidovudine is a synthetic thymidine analogue.
- Mechanism: It is converted in vivo by cellular kinases into its active metabolite, zidovudine triphosphate.
- Pathway: Zidovudine triphosphate competes with natural thymidine triphosphate for incorporation into viral DNA by viral RNA-directed DNA polymerase (→ reverse transcriptase).
- Effect: Because zidovudine lacks a 3'-OH group, its incorporation prevents the formation of phosphodiester linkages, leading to DNA chain termination and a virustatic effect against retroviruses.
- It also possesses some activity against gram-negative bacteria and can exhibit host cell cytotoxicity (particularly in rapidly dividing cells like bone marrow).
Safety & warnings
Contraindications
- Patients with a history of life-threatening hypersensitivity reactions to zidovudine
Adverse effects
- Non-regenerative anemia (most common in cats, dose-dependent)
- Reductions in RBCs, PCV, and hemoglobin
- Diarrhea
- Weakness
- Granulocytopenia (reported in humans)
- Gastrointestinal effects
Precautions
Bone Marrow Suppression: Use with extreme caution in patients with pre-existing bone marrow dysfunction. Cats with bone marrow suppression should generally not be treated with zidovudine.
- Organ Dysfunction: Dosage adjustment may be necessary in cats with renal or hepatic dysfunction.
- Pregnancy: FDA Category C for use during pregnancy (animal studies show adverse fetal effects, but adequate human studies are lacking).
- Lactation: Excreted in milk; clinical significance for nursing offspring is unclear.
Drug interactions
May increase zidovudine levels
May increase zidovudine levels
May antagonize each other's effects; avoid concurrent use. Also increases risk of hematologic toxicity.
Increased risk for hematologic and hepatotoxicity
May increase zidovudine levels
Administered with zidovudine may increase the risk of hematologic toxicity
May decrease blood levels (AUC) of zidovudine
Monitoring
- CBC (Complete Blood Count): Weekly for the first month of treatment (non-regenerative anemia is common, especially at higher doses). If stable, monthly checks are sufficient.
- Hematocrit: If PCV drops below 20%, treatment should be discontinued (anemia usually resolves within a few days).
- CD4/CD8 ratios (if possible)
- Clinical efficacy and improvement of signs
Pharmacokinetics
Half-life
Absorption
Well absorbed after oral administration. In cats, oral bioavailability is approximately 95%. When administered with food, peak levels may be decreased, but total AUC is generally unaffected. Peak levels occur about 1 hour post-dosing in cats.
Distribution
Widely distributed throughout the body, including penetration into the cerebrospinal fluid (CSF). It is only marginally bound to plasma proteins.
Metabolism
Rapidly metabolized in vivo.
Elimination
Excreted primarily in the urine.
Overdose
Human adults and children have survived massive oral overdoses (up to 50 grams) without permanent sequelae.
- Clinical Signs: Vomiting and transient hematologic effects (e.g., bone marrow suppression) are the most consistent adverse effects reported with acute overdoses.
- Management: Treatment is largely supportive and symptomatic, focusing on gastrointestinal decontamination if caught early, and monitoring hematologic parameters.
Available products
Formulations
- Tablets
- Capsules
- Syrup
- Injection solution
Human-labeled
- Zidovudine Oral Tablets: 300 mg (Retrovir, generic)
- Zidovudine Oral Capsules: 100 mg (Retrovir, generic)
- Zidovudine Oral Syrup: 50 mg/5 mL in 240 mL (Retrovir, generic)
- Zidovudine Injection Solution: 10 mg/mL in 20 mL single-use vials (Retrovir)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Oral tablets, capsules, and syrup should be stored at room temperature. Protect from heat, light, and moisture. Injection (for IV infusion) should be stored at room temperature and protected from light.
Client information
- Experimental Therapy: This medication is considered an 'experimental' or off-label therapy for cats with viral infections.
- Strict Compliance: It must be administered exactly as prescribed by your veterinarian to maintain efficacy against the virus.
- Mandatory Blood Tests: Regular veterinary visits for blood tests are absolutely required (often weekly at first) because this drug can cause severe anemia.
- Watch for Side Effects: Contact your veterinarian immediately if your cat becomes unusually weak, lethargic, develops pale gums, or stops eating.
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.