Fluoxetine Hydrochloride — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	1-2 mg/kg	q24h	Duration: Long-term; may take 2-4 weeks for full effect Notes: Start at lower end; adjust based on response. For separation anxiety, often used with behavior modification.
Cat	PO	0.5-1 mg/kg	q24h	Duration: Long-term; may take 2-4 weeks for full effect Notes: Use lower dose initially. For urine marking, typical dose is 0.5-1 mg/kg once daily.
Horse	PO	0.1-0.5 mg/kg	q24h	Duration: Variable; monitor for efficacy Notes: Limited data; use cautiously. May be compounded.
Rabbit	PO	0.5-2 mg/kg	q24h	Duration: Variable Notes: Extrapolated from other species; use with caution.
Bird (e.g., psittacines)	PO	1-2 mg/kg	q24h	Duration: Variable Notes: Limited studies; use with caution.

Clinical Indications & Species Uses

General Indications

Behavioral disorders including anxiety, phobias, and compulsive behaviors

Dog (Canine)

Separation anxiety
Generalized anxiety disorders
Compulsive behaviors (e.g., excessive licking, tail chasing)
Aggression (as adjunct to behavior modification)
Canine cognitive dysfunction (off-label)

Cat (Feline)

Urine marking/spraying
Feline idiopathic cystitis (as adjunct)
Compulsive behaviors (e.g., overgrooming)
Anxiety-related aggression
Social anxiety

Pharmacology & Mechanism of Action

Drug Class: Selective Serotonin Reuptake Inhibitor (SSRI) | Pharmacological Group: Antidepressant / Psychotropic Agent

Mechanism of Action: Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) that blocks the reuptake of serotonin (5-HT) at the presynaptic neuronal membrane, thereby increasing the concentration of serotonin in the synaptic cleft. This enhances serotonergic neurotransmission. Unlike many other SSRIs, fluoxetine has a long-acting active metabolite, norfluoxetine, which contributes to its prolonged duration of action. It has minimal affinity for other receptors such as muscarinic, histaminergic, or adrenergic receptors, which reduces certain side effects.

Pharmacodynamics: Fluoxetine enhances serotonergic activity in the central nervous system, which modulates mood, anxiety, and compulsive behaviors. In veterinary medicine, it is used to manage behavioral disorders such as separation anxiety, aggression, and compulsive behaviors. The therapeutic effect typically develops over 2-4 weeks due to receptor downregulation and adaptive changes. It may also have mild anorexigenic effects and can influence appetite and weight.

⚡ Pharmacokinetics Summary

Absorption: Fluoxetine is well absorbed after oral administration. Peak plasma concentrations occur approximately 1-6 hours after dosing in dogs and cats. Food does not significantly affect absorption.

Distribution: Fluoxetine is widely distributed throughout the body, with high affinity for tissues. It is highly lipophilic and accumulates in the brain. The volume of distribution is large. It crosses the blood-brain barrier and placenta and is excreted in milk.

Metabolism: Fluoxetine is extensively metabolized in the liver primarily by cytochrome P450 enzymes (CYP2D6 in humans; in animals, similar isoenzymes). The primary metabolite is norfluoxetine, which is pharmacologically active and has a longer half-life than the parent compound. Metabolism may be slower in cats due to reduced glucuronidation capacity.

Excretion: Fluoxetine and its metabolites are excreted primarily in the urine (approximately 60-80%) and feces (approximately 15%). In dogs, the elimination half-life of fluoxetine is approximately 6-9 hours, but norfluoxetine has a half-life of 1-3 days. In cats, the half-life is longer, with fluoxetine around 24 hours and norfluoxetine around 2-4 days.

Half-Life: Dog: fluoxetine 6-9 h, norfluoxetine 1-3 days; Cat: fluoxetine ~24 h, norfluoxetine 2-4 days; Horse: fluoxetine ~2-4 h (short), but norfluoxetine may be longer.

Bioavailability: Oral bioavailability is high (approximately 60-80% in dogs and cats) due to first-pass metabolism.

Protein Binding: Fluoxetine is approximately 94% protein-bound in plasma; norfluoxetine is about 90% bound.

Available Formulations & Strengths

Oral Tablet 10 mg, 20 mg, 40 mg (PO)

Oral Capsule 10 mg, 20 mg, 40 mg (PO)

Oral Solution 20 mg/5 mL (PO)

Compounded Suspension Various (e.g., 10 mg/mL) (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to fluoxetine or any component
Concurrent use with MAO inhibitors (e.g., selegiline) - risk of serotonin syndrome
Use within 14 days of MAO inhibitor therapy
Severe hepatic impairment (use with caution)
History of seizures (may lower seizure threshold)

Warnings & Clinical Precautions:

Use with caution in patients with hepatic or renal disease; dose adjustment may be necessary.
May cause anorexia and weight loss; monitor body condition.
May increase anxiety or agitation in some animals initially.
Use with caution in aggressive animals; may paradoxically increase aggression in some cases.
Do not discontinue abruptly; taper dose to avoid withdrawal signs.
Safety in pregnant or lactating animals has not been established; use only if benefits outweigh risks.
May interact with other serotonergic drugs (e.g., tramadol, amitriptyline) - risk of serotonin syndrome.
In cats, half-life is prolonged; adjust dosing interval if needed.

Adverse Effects & Reactions

Common:

Anorexia
Lethargy
Sedation
Vomiting
Diarrhea
Restlessness
Insomnia
Decreased appetite

Serious / Severe:

Serotonin syndrome (hyperthermia, tremors, agitation, seizures)
Seizures
Hepatic toxicity (rare)
Aggression (paradoxical)
Cardiac arrhythmias (rare)

Rare:

Priapism
Mania-like behavior
Extrapyramidal signs
Photosensitivity

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
MAO inhibitors (e.g., selegiline)	Severe serotonin syndrome (hyperthermia, tremors, seizures) - contraindicated	High
Other SSRIs or SNRIs (e.g., paroxetine, venlafaxine)	Additive serotonergic effects, increased risk of serotonin syndrome	High
Tramadol	Increased risk of serotonin syndrome	Moderate
Tricyclic antidepressants (e.g., amitriptyline, clomipramine)	Increased plasma levels of TCAs, risk of toxicity and serotonin syndrome	High
NSAIDs (e.g., carprofen, meloxicam)	Increased risk of gastrointestinal bleeding (serotonin affects platelet function)	Moderate
Cytochrome P450 inhibitors (e.g., ketoconazole)	May increase fluoxetine levels	Moderate
Cytochrome P450 inducers (e.g., phenobarbital)	May decrease fluoxetine levels	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Vomiting
Lethargy
Agitation
Tremors
Seizures
Hyperthermia
Tachycardia
Mydriasis
Serotonin syndrome

Emergency Treatment Protocol: Treatment is primarily supportive. Induce emesis if recent ingestion and no contraindications. Administer activated charcoal to reduce absorption. Provide symptomatic and supportive care: IV fluids, temperature management, anticonvulsants (e.g., diazepam) for seizures, and cyproheptadine (a serotonin antagonist) for serotonin syndrome (dose: 1.1 mg/kg in dogs, 2-4 mg/cat, PO or rectally). Monitor vital signs and cardiac function. Hospitalization may be required.

Food Animal Withdrawal Times

Not approved for food animals. Withdrawal times not established. Do not use in animals intended for human consumption.

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).

Light Sensitivity: Light-sensitive — protect from direct exposure.

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

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for human use; not FDA-approved for veterinary use, but commonly used off-label in dogs and cats.

Extra-Label (Off-Label) Use: In the US, extra-label use in food animals is prohibited. For non-food animals, extra-label use is permitted under AMDUCA with a valid veterinarian-client-patient relationship. In the EU, similar regulations apply.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Fluoxetine is a valuable tool for managing behavioral disorders in dogs and cats, particularly separation anxiety and urine marking. It is important to combine pharmacotherapy with behavior modification for optimal outcomes. The onset of action is delayed (2-4 weeks), so patience and client education are essential. Doses should be individualized, and animals should be monitored for adverse effects, especially during the first few weeks. Abrupt discontinuation should be avoided. In cats, the long half-life allows for every-other-day dosing in some cases, but daily dosing is standard. Use with caution in animals with hepatic or renal impairment. Always rule out medical causes of behavioral signs before initiating treatment.

References & Literature

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