Lamivudine
2',3'-dideoxy-3'-thiacytidine
Also known as: Epivir · Zeffix · 3TC · Lamivudinum
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 |
|---|---|---|---|---|---|
| Treatment of FIV-positive cats | 25 mg/kg | PO | q12h | — | 🌍 EU |
- Treatment of FIV-positive cats: May be combined with zidovudine for enhanced efficacy.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Lamivudine (also known as 3TC) is an antiviral medication primarily used in veterinary medicine for the management of Feline Immunodeficiency Virus (FIV). In asymptomatic cats, it may delay the onset of the clinical phase. In symptomatic cats, it is often used as part of a combination therapy to improve recovery.
Clinical Warning: Lamivudine can cause bone marrow suppression; regular hematological monitoring is essential.
Clinical Pearl: Combining lamivudine with zidovudine (AZT) has been shown to be more effective at reducing viral load and improving CD4/CD8 ratios over a 1-year period than zidovudine monotherapy.
Mechanism of action
Lamivudine is a nucleoside reverse transcriptase inhibitor (NRTI). It acts as an analogue of cytidine. Once phosphorylated to its active triphosphate form intracellularly, it competes with natural substrates and incorporates into viral DNA. This causes chain termination → inhibits the viral reverse transcriptase enzyme → prevents viral replication.
Safety & warnings
Contraindications
- Anaemia
- Pre-existing severe bone marrow suppression
Adverse effects
- Cytopenias (predicted)
- Anaemia
- Leukopenia
Precautions
Haematological monitoring is recommended due to the predictable risk of cytopenias.
Normal precautions should be observed for safety and handling of antiviral medications.
Drug interactions
Synergistic therapeutic effect; more effective at reducing viral load and increasing CD4/CD8 ratios than zidovudine alone.
Monitoring
- Complete Blood Count (CBC) regularly to monitor for cytopenias
- CD4/CD8 ratios
- Viral load
Pharmacokinetics
Absorption
Well absorbed orally in most mammalian species.
Distribution
Distributes widely into total body fluid; penetrates the blood-brain barrier.
Metabolism
Undergoes minimal hepatic metabolism.
Elimination
Primarily excreted unchanged in the urine.
Overdose
Information on overdosage in veterinary species is limited. Overdose is likely to exacerbate hematological toxicities such as severe cytopenias (anemia, leukopenia). Treatment should be supportive and symptomatic.
Available products
Formulations
- 10 mg/ml oral solution
- 100 mg tablets
Human-labeled
- Epivir 10 mg/ml oral solution
- Epivir 100 mg tablets
- Zeffix 100 mg tablets
Regulatory status
POM (Prescription Only Medicine). Human formulations used off-label under the cascade.
POM (Prescription Only Medicine). Human formulations used off-label under the cascade.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store at room temperature (15-30°C), protected from moisture and light. Keep oral solutions tightly closed.
Client information
Lamivudine is used to help manage FIV (Feline Immunodeficiency Virus) in cats. It is not a cure, but it can help lower the amount of virus in your cat's body and support their immune system.
- Give exactly as prescribed: Do not skip doses, as this can lead to viral resistance.
- Monitor for side effects: Watch for signs of anemia or low white blood cells, such as extreme tiredness, pale gums, or unexpected infections. Contact your vet if you notice these.
- Follow-up appointments: Your vet will need to run regular blood tests to ensure the medication is not negatively affecting your cat's bone marrow.
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.