Trazodone Hydrochloride

Antidepressant (Serotonin Modulator) Phenylpiperazine antidepressant; Serotonin 5-HT2A receptor antagonist and serotonin reuptake inhibitor

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 2.5-5 mg/kg (up to 10 mg/kg for severe anxiety) q8-24h (usually q12h for chronic use; q8h for situational anxiety) Duration: As needed or for 1-4 weeks depending on condition
Notes: Start at lower end for sedation; may combine with other behavioral medications. For situational anxiety, give 1-2 hours prior to event.
Cat PO 2.5-5 mg/kg (up to 7.5 mg/kg) q12-24h Duration: As needed or for 1-4 weeks
Notes: Cats may require higher doses; monitor for sedation. For situational anxiety, give 1-2 hours prior.
Horse PO 0.5-1 mg/kg (up to 2 mg/kg) q12-24h Duration: As needed
Notes: Limited data; use with caution. May be used for transport or handling anxiety.
Rabbit PO 5-10 mg/kg q12-24h Duration: As needed
Notes: Limited data; use with caution.

Clinical Indications & Species Uses

General Indications
  • Management of anxiety and stress-related behaviors in companion animals
Dog (Canine)
  • Anxiety disorders (separation anxiety, noise phobias, situational anxiety)
  • Sedation for veterinary visits or procedures
  • Adjunct to behavioral modification
  • Post-operative sedation
  • Treatment of aggression (as adjunct)
Cat (Feline)
  • Anxiety and fear-related behaviors
  • Sedation for transport or veterinary visits
  • Adjunct to behavioral modification
  • Urine spraying (off-label)
Horse (Equine)
  • Anxiolysis and sedation for handling, transport, or procedures
  • Adjunct to behavioral modification in horses
Cattle (Bovine)
  • Potential for sedation in handling (off-label)
Rabbit & Small Mammals
  • Anxiolysis and sedation for handling (off-label)
Exotic & Other Species
  • Anxiolysis in some exotic species (off-label, limited data)

Pharmacology & Mechanism of Action

Drug Class: Antidepressant (Serotonin Modulator) | Pharmacological Group: Phenylpiperazine antidepressant; Serotonin 5-HT2A receptor antagonist and serotonin reuptake inhibitor

Mechanism of Action: Trazodone hydrochloride is a triazolopyridine derivative that acts primarily as an antagonist at serotonin 5-HT2A receptors and, to a lesser extent, inhibits serotonin reuptake. It also has antagonist activity at histamine H1 and alpha1-adrenergic receptors, contributing to its sedative and anxiolytic effects. The net effect is modulation of serotonergic neurotransmission, leading to reduced anxiety and agitation without significant anticholinergic effects.

Pharmacodynamics: In veterinary species, trazodone produces dose-dependent sedation, anxiolysis, and muscle relaxation. It reduces fear, anxiety, and stress-related behaviors. Its antagonism at 5-HT2A receptors is thought to underlie its anxiolytic and antidepressant effects, while alpha1-adrenergic blockade contributes to hypotension and sedation. The drug has a wide therapeutic index and is commonly used for short-term management of situational anxiety and as an adjunct to behavioral modification.

⚡ Pharmacokinetics Summary

Absorption: Trazodone is well absorbed after oral administration in dogs and cats. Peak plasma concentrations occur approximately 1-2 hours after dosing in dogs and 1-3 hours in cats. Food can increase absorption and reduce first-pass metabolism.
Distribution: Trazodone is widely distributed throughout the body, with high affinity for tissues. It crosses the blood-brain barrier and placenta. The volume of distribution is large, approximately 1-2 L/kg in dogs. Protein binding is moderate to high (89-95%) in humans, but species-specific data are limited.
Metabolism: Trazodone undergoes extensive hepatic metabolism, primarily via cytochrome P450 enzymes (CYP3A4 in humans, but species-specific isoenzymes are involved). The major active metabolite is meta-chlorophenylpiperazine (mCPP), which has serotonergic activity. In dogs, metabolism is rapid, with mCPP formation being significant.
Excretion: Metabolites are excreted primarily in the urine (75%) and feces (20%). In dogs, the elimination half-life is approximately 2-3 hours, while in cats it is approximately 4-6 hours. Clearance is hepatic and renal.
Half-Life: Dog: 2-3 hours; Cat: 4-6 hours; Horse: approximately 2-4 hours (limited data)
Bioavailability: Oral bioavailability is approximately 65% in dogs (due to first-pass metabolism) and may be higher in cats. Food increases bioavailability.
Protein Binding: 89-95% (human data; likely similar in dogs and cats)

Available Formulations & Strengths

Oral Tablet 50 mg, 100 mg, 150 mg, 300 mg (PO)
Oral Capsule 50 mg, 100 mg (PO)
Oral Solution (compounded) Various (e.g., 10 mg/mL) (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to trazodone or any component
  • Concurrent use with MAO inhibitors (e.g., selegiline) - risk of serotonin syndrome
  • Severe hepatic or renal impairment (use with caution)
  • Cardiac arrhythmias, especially QT prolongation
  • History of priapism (in males)
  • Concurrent use with other serotonergic drugs (e.g., SSRIs, SNRIs) - risk of serotonin syndrome
Warnings & Clinical Precautions:
  • Use with caution in patients with cardiovascular disease, hypotension, or dehydration
  • May cause sedation and ataxia; monitor for excessive sedation
  • Use with caution in patients with a history of seizures
  • In cats, may cause increased vocalization or paradoxical excitation
  • Do not use in pregnant or lactating animals unless benefits outweigh risks (limited safety data)
  • Use with caution in geriatric animals; may require dose reduction
  • Avoid abrupt discontinuation after prolonged use; taper dose
  • Monitor for signs of serotonin syndrome (agitation, tremors, hyperthermia) when used with other serotonergic drugs

Adverse Effects & Reactions

Common:

  • Sedation
  • Lethargy
  • Vomiting
  • Diarrhea
  • Anorexia
  • Ataxia

Serious / Severe:

  • Serotonin syndrome (when combined with other serotonergic drugs)
  • Cardiac arrhythmias (QT prolongation)
  • Hypotension
  • Priapism (in males)
  • Seizures (rare)

Rare:

  • Hepatotoxicity
  • Blood dyscrasias
  • Allergic reactions
  • Paradoxical excitation (especially in cats)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
MAO inhibitors (e.g., selegiline) Increased risk of serotonin syndrome (hyperthermia, agitation, tremors) High
SSRIs/SNRIs (e.g., fluoxetine, clomipramine) Additive serotonergic effects, risk of serotonin syndrome High
Tramadol Additive serotonergic effects, risk of serotonin syndrome Moderate
CNS depressants (e.g., opioids, benzodiazepines, phenothiazines) Additive sedation and respiratory depression Moderate
Antihypertensives (e.g., ACE inhibitors, beta-blockers) Additive hypotensive effects Moderate
CYP3A4 inhibitors (e.g., ketoconazole, erythromycin) Increased trazodone plasma levels, risk of toxicity Moderate
CYP3A4 inducers (e.g., phenobarbital) Decreased trazodone plasma levels, reduced efficacy Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Severe sedation
  • Ataxia
  • Vomiting
  • Hypotension
  • Tachycardia or bradycardia
  • Respiratory depression
  • Seizures
  • Coma
  • QT prolongation and arrhythmias

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce emesis if recent ingestion and patient is conscious. Administer activated charcoal to reduce absorption. Provide intravenous fluids for hypotension. Monitor cardiac function (ECG) and respiratory status. Use benzodiazepines for seizures. In severe cases, consider serotonin antagonists (e.g., cyproheptadine) if serotonin syndrome is suspected. There is no specific antidote.

Food Animal Withdrawal Times

Not approved for food animals; no withdrawal times established. Extra-label use in food animals is prohibited or requires extended withdrawal periods; consult regulatory authorities.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F); excursions permitted between 15-30°C (59-86°F)

Handling & Special Conditions: Protect from moisture. Keep container tightly closed. Compounded formulations may have different storage requirements; follow label.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use (e.g., Desyrel).

Extra-Label (Off-Label) Use: In the US, trazodone is not FDA-approved for veterinary use; it is used extra-label under AMDUCA. For food animals, extra-label use is prohibited if an approved animal drug exists that is effective, and withdrawal times must be extended. In companion animals, extra-label use is common and accepted.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Trazodone is a valuable adjunct for managing anxiety and stress in veterinary patients. It is particularly useful for situational anxiety (e.g., thunderstorms, fireworks, veterinary visits) and as a short-term sedative. In dogs, it is often used in combination with behavioral modification and other medications (e.g., fluoxetine) for chronic anxiety. In cats, it can be used for fear-related aggression and transport anxiety. Dosing should be individualized based on response and tolerance. Monitor for excessive sedation, especially in geriatric patients. Avoid concurrent use with other serotonergic drugs to prevent serotonin syndrome. For food animals, use is not recommended due to lack of withdrawal data. Always obtain a thorough history and perform a physical examination before prescribing.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)