Lysine
L-2,6-diaminohexanoic acid
Also known as: Viralys · Enisyl · Incremin · LLysine Powder-Pure · L-lysine · K · Lysine hydrochloride
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 |
|---|---|---|---|---|---|
| To prevent or reduce recurrent feline herpesvirus ocular infections | 500 mg PO twice daily for life | PO | q12h | for life | 🌎 NA |
| To prevent or reduce recurrent feline herpesvirus ocular infections | 500 mg mixed with food daily | PO | q24h | — | 🌎 NA |
| To prevent or reduce recurrent feline herpesvirus ocular infections | 250 mg PO twice daily | PO | q12h | — | 🌎 NA |
| As adjunctive therapy for feline herpesvirus dermatologic infections | 250 mg PO twice daily | PO | q12h | — | 🌎 NA |
| As adjunctive therapy for feline herpesvirus dermatologic infections | 250 mg PO once to twice daily | PO | q12h-q24h | — | 🌎 NA |
| Feline herpesvirus-1 (FHV-1) infection management | 500 mg | PO | q12-24h | — | 🌍 EU |
| Feline herpesvirus-1 (FHV-1) infection management | 250 mg | PO | q12-24h | — | 🌍 EU |
- Feline herpesvirus-1 (FHV-1) infection management: Adults. Equivalent to 2 ml/2 pumps q12h.
- Feline herpesvirus-1 (FHV-1) infection management: Kittens. Equivalent to 1 ml/1 pump q12h.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Lysine is a nutritional amino acid traditionally used in veterinary medicine with the intent of suppressing Feline Herpesvirus-1 (FHV-1) infections in cats.
Clinical Controversy: While historically popular, recent robust studies have cast significant doubt on its efficacy. Research indicates that lysine supplementation does not prevent or reduce the recurrence of upper respiratory tract infections in shelter cats. In some studies, cats receiving lysine actually showed an increase in disease severity and FHV-1 DNA detection. Consequently, many veterinary ophthalmologists and infectious disease experts no longer recommend its routine use.
Mechanism of action
Lysine is an amino acid that acts as a competitive antagonist to arginine during viral replication.
- Mechanism: Herpesviruses require arginine to synthesize viral proteins and produce infective viral particles.
- Pathway: Lysine competes with arginine → Lysine is erroneously incorporated into the viral structure instead of arginine → The resulting viral particles are defective and less infective.
Note: The clinical failure of this mechanism in cats may be due to the feline's strict dietary requirement for arginine; artificially lowering arginine activity via lysine supplementation can have detrimental systemic effects.
Safety & warnings
Contraindications
- No specific contraindications noted
- Preparations containing propylene glycol (toxic to cats)
Adverse effects
- Abdominal pain
- Diarrhea
- Nausea
- Vomiting
- Diarrhoea (mild, reversible)
Precautions
There are no specific contraindications, but caution is advised regarding its clinical utility. Recent evidence suggests it may lack efficacy for FHV-1 and could potentially exacerbate upper respiratory disease in shelter environments. Safety in pregnant animals other than rats has not been established.
Drug interactions
May negate the anti-herpesvirus effects of lysine
Concomitant use may increase calcium absorption from the gut and decrease calcium loss in the urine
Monitoring
- Clinical signs of herpesvirus infection (ocular and respiratory symptoms)
- Clinical signs of FHV-1 (ocular and respiratory signs)
- Gastrointestinal tolerance (monitoring for diarrhoea)
Overdose
Significant toxicity is highly unlikely.
- Clinical Signs: Mild gastrointestinal effects such as nausea, vomiting, and diarrhea may occur with massive ingestions.
- Management: Supportive care as needed; usually self-limiting.
Available products
Formulations
- Oral gel
- Oral powder
- Oral tablets
- Oral capsules
- 250 mg/ml oral paste
- 250 mg oral capsule
- 500 mg oral capsule
Veterinary
- Viralys Gel: 250 mg per 1.25 mL
- Viralys Powder: 250 mg per rounded scoop
- LLysine Powder-Pure Feed Additive (for horses)
- Enisyl 250 mg/ml oral paste (100 ml bottle)
- 250 mg capsules
- 500 mg capsules
Human-labeled
- Enisyl Tablets & Capsules: 312 mg, 334 mg, 500 mg & 1000 mg
- Various generic OTC tablets and capsules (250 mg to 1000 mg)
Regulatory status
General sale.
General sale.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Unless otherwise specified on the label, lysine should be stored at room temperature in tight containers.
Client information
Administration Tips
- Lysine is easiest to administer by crushing tablets or emptying capsules and mixing the powder directly into your cat's wet food.
- Adverse stomach effects are very unlikely when given with food.
What to Expect
- Lysine does not cure the herpesvirus infection. If effective, it only helps to control it by reducing the severity and frequency of flare-ups.
- Lifetime therapy may be required to manage chronic infections.
Important Warning: If you are purchasing over-the-counter human liquid formulations, avoid products that contain propylene glycol, as this can be toxic to cats.
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.