Wondering, “How long does chlorpromazine last in horses?” As a pharmacist with expertise in medications like chlorpromazine—commonly known as Thorazine or Largactil—I’m here to shed light on its duration in equine use. While it’s a first-generation antipsychotic in humans, chlorpromazine is sometimes used off-label in veterinary medicine, including horses, for sedation or behavioral calming. Its duration depends on dosage, administration, and equine metabolism. Let’s dive in.
What Is Chlorpromazine in Horses?
Chlorpromazine is a phenothiazine drug that blocks dopamine D2 receptors, with sedative effects from histamine antagonism. In horses, it’s not a primary sedative like acepromazine (another phenothiazine), but vets may use it off-label for agitation, anxiety, or severe behavioral issues. It’s typically given as an intramuscular (IM) injection or, rarely, orally (chlorpromazine 25mg equivalent adjusted for weight).
Quick Note: Unlike its human uses (chlorpromazine for hiccups, schizophrenia), equine applications lack extensive BNF chlorpromazine-style guidelines, so dosing is vet-specific.
How Long Does Chlorpromazine Last in Horses?
Here’s the breakdown of its duration in horses, based on pharmacokinetics and veterinary practice:
Duration of Effect
- Onset: 15-30 minutes (IM injection).
- Peak Effect: 1-2 hours.
- Duration: 4-12 hours, depending on dose and horse factors.
- Why: Chlorpromazine sedates via histamine and dopamine blockade, with effects fading as it’s metabolized by the liver.
In my experience with similar drugs, a typical equine dose (e.g., 0.4-1 mg/kg IM) provides sedation for 4-8 hours, though some horses show residual calmness up to 12 hours.
Half-Life in Horses
- Estimated Half-Life: 6-12 hours (based on human data adjusted for equine metabolism).
- Clearance: Fully cleared from the system in 24-48 hours, though detectable traces may linger longer.
- Why: Horses metabolize drugs faster than humans, but chlorpromazine’s lipophilic nature prolongs its action.
Timeline Table
| Aspect | Timeframe | Notes |
|---|---|---|
| Onset | 15-30 min | Faster with IM vs. oral |
| Peak Sedation | 1-2 hrs | Strongest calming effect |
| Duration of Action | 4-12 hrs | Dose-dependent |
| Full Clearance | 24-48 hrs | Metabolized by liver |
Factors Affecting Duration
- Dosage: Higher doses (e.g., 1 mg/kg vs. 0.4 mg/kg) extend duration (chlorpromazine dosage).
- Route: IM lasts longer than oral due to better absorption in horses.
- Horse Factors: Age, weight (e.g., 500 kg average), and liver function alter metabolism.
- Condition: Agitation may shorten perceived effects as the horse fights sedation.
Vet Tip: A 500 kg horse might get 200-500 mg IM, lasting 6-10 hours, but always consult a vet.
Uses in Horses
- Sedation: Calms aggressive or anxious horses (e.g., during transport).
- Behavioral Control: Manages extreme stress or stereotypies (off-label).
- Not for Hiccups: Unlike humans (chlorpromazine for hiccups), this isn’t a equine use.
Caution: It’s less common than acepromazine due to risks like hypotension (side effects of chlorpromazine).
Side Effects to Watch
- Common: Drowsiness, slow movement.
- Serious: Low blood pressure, extrapyramidal symptoms (rare in horses), or prolonged sedation.
- Duration Impact: Side effects may outlast benefits if dosing is off.
How Does Chlorpromazine Work in Horses?
It sedates by blocking dopamine and histamine receptors, reducing nervous system excitability (how does chlorpromazine work). In horses, this translates to a calmer state for hours, not days.
Final Thoughts
So, how long does chlorpromazine last in horses? Typically, 4-12 hours of sedation, peaking at 1-2 hours, with full clearance by 24-48 hours. It’s a potent option for equine calming, but its off-label status means vet oversight is key. Got a horse on chlorpromazine? Share your experience below—I’d love to hear!