Vetoquinol · Review
Zylkene Plus
Vetoquinol's Zylkene is the only US-available supplement built around alpha-casozepine — a hydrolyzed milk peptide with the strongest non-L-theanine canine evidence base. It's a daily-use product for sustained calming, not a same-day rescue.
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The most evidence-backed daily calming supplement on US shelves
Zylkene is one of only two non-prescription calming supplements with meaningful canine clinical evidence (the other being L-theanine). The 2007 Beata trial compared alpha-casozepine head-to-head against selegiline (Anipryl, a prescription MAOI) over 56 days and found no statistically significant difference in EDED anxiety scores or owner ratings. That benchmark is unusual in this category.
The flip side: it does not work in the short term. Plan ahead — load Zylkene 5–7 days before a known stressor (vet visit, fireworks week, travel). For acute panic, reach for L-theanine or a vet-prescribed anxiolytic instead.
Pros
- ✓Strongest non-L-theanine evidence base in canine supplements
- ✓Compared favorably to prescription selegiline in published trial
- ✓No sedation, no known drug interactions at label dose
- ✓Lactose-free despite milk-protein origin
- ✓Capsule can be opened and sprinkled on food
Cons
- ✗Takes 5–7 days minimum to build effect — useless for tonight's storm
- ✗More expensive per dose than most calming chews
- ✗Not for dogs with confirmed dairy-protein hypersensitivity
- ✗Capsules can be tricky for owners who prefer chews
How it works
Alpha-casozepine is a decapeptide derived from bovine αs1-casein (a milk protein). It binds GABA-A receptors at a site distinct from benzodiazepines — producing anxiolysis without sedation, ataxia, or muscle relaxation. Daily administration over weeks appears to reduce baseline anxiety responses; effect builds gradually rather than spiking.
The evidence
The Beata 2007 trial (38 anxious dogs, 56-day blinded comparison vs selegiline) is the foundational study. Subsequent published trials (2010 placebo-controlled, 2025 Frontiers in Veterinary Science vet-visit study) reinforce the signal. Edinburgh's 2017 systematic review (Buckley) concluded "limited and weak evidence to suggest it may have a role in reducing anxiety in dogs over the medium to longer term."
Best for
- Dogs with chronic, low-grade generalized anxiety
- Pre-loading before predictable events (board, fireworks week, travel)
- Multi-anxiety dogs where daily baseline reduction matters more than acute relief
- Dogs already on prescription medication where adding L-tryptophan would risk serotonin syndrome
Not the right fit if…
- Acute single-event anxiety (no time to build effect)
- Severe noise phobia or panic that needs prescription
- Dogs with diagnosed dairy-protein allergy
- Owners who can't commit to consistent daily dosing
How to use
Capsule once daily with food — start 5–7 days before a known stressor and continue through the period. Vetoquinol's dosing chart: 75 mg/day for cats and dogs under 22 lb; 225 mg for 22–65 lb; 450 mg for over 65 lb. Capsules can be opened and contents sprinkled if your dog won't swallow whole.
For chronic anxiety, daily long-term use is well tolerated. There's no published evidence of tolerance buildup or rebound on discontinuation.
Price
$30–$60 / month depending on dog size
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