Promethazine
Promethazine is a phenothiazine derivative utilized in veterinary medicine primarily as an antiemetic and antihistamine. * **Primary Uses**: It is occasionally used to manage nausea and vomiting in dogs and cats, and has been trialed for pruritus in atopic dogs (though efficacy for the latter is generally poor). * **Pharmacological Profile**: While structurally related to tranquilizers like acepromazine, promethazine is distinguished by its potent antihistaminic properties alongside its antiemetic, anticholinergic, sedative, and mild local anesthetic effects. > **Clinical Pearl**: Promethazine is relatively uncommonly prescribed in modern veterinary practice compared to newer, more targeted antiemetics (e.g., maropitant, ondansetron) or standard antihistamines (e.g., diphenhydramine), largely due to its potential for sedation and extrapyramidal side effects.
Mecanismo: Promethazine exerts its effects through competitive antagonism at multiple receptor sites centrally and peripherally: * **H1-Receptor Antagonism**: Competitively blocks histamine at H1 receptors → reduces allergic responses and pruritus. * **D2-Dopaminergic Antagonism**: Blocks dopamine receptors in the Chemoreceptor Trigger Zone (CRTZ) of the medulla → provides central **antiemetic** effects. * **Muscarinic Antagonism**: Blocks acetylcholine at muscarinic receptors → contributes to antiemetic efficacy and causes anticholinergic side effects (e.g., dry mouth). * **Alpha-1 Adrenergic Antagonism**: Blocks peripheral alpha-1 receptors → can lead to vasodilation and subsequent hypotension.
Dosificación por especie
- As an antiemetic · 2 mg/kg PO or IM once daily · PO/IM · q24h
- As an antihistamine · 0.2-0.4 mg/kg PO three to four times a day · PO · q6-8h
- As an antihistamine · 1-2 mg/kg PO q12h · PO · q12h
- As an antiemetic · 2 mg/kg PO or IM once daily · PO/IM · q24h
- As an antihistamine · 0.2-0.4 mg/kg PO three to four times a day · PO · q6-8h
- As an antihistamine · 1-2 mg/kg PO q12h · PO · q12h
Las dosis son una referencia clínica para veterinarios colegiados. Confirme siempre con la ficha técnica vigente y el paciente individual.
Vías de administración
Contraindicaciones
- Hypovolemia or shock (due to hypotensive effects)
- Tetanus or strychnine intoxication (due to extrapyramidal effects)
- Human label black box warning: Contraindicated in children < 2 years old (fatal respiratory depression)
- Known hypersensitivity to phenothiazines
- Severe central nervous system depression
- Comatose patients
- Use with caution in patients with seizure disorders
- Use with caution in patients with severe hepatic or cardiovascular disease
Efectos adversos
- Sedation or lethargy
- Anticholinergic effects (dry mouth, urinary retention, decreased GI motility)
- Hypotension (especially with rapid IV administration or in hypovolemic patients)
- Extrapyramidal signs (tremors, rigidity, torticollis) - rare but possible
- Sedation / Lethargy
- Dry mucous membranes (anticholinergic effect)
- Tachycardia
- Hypotension
- Paradoxical excitation (especially in cats)
- Gastrointestinal disturbances
Interacciones farmacológicas
- Antacids · May cause reduced GI absorption of oral phenothiazines.
- Antidiarrheal mixtures (e.g., Kaolin/pectin, bismuth subsalicylate) · May cause reduced GI absorption of oral phenothiazines.
- Atropine & other anticholinergics · May have additive anticholinergic effects when used with promethazine.
- CNS Depressant Agents (barbiturates, narcotics, anesthetics) · May cause additive CNS depression if used with phenothiazines.
- Epinephrine · Phenothiazines block alpha-adrenergic receptors; concomitant epinephrine can lead to unopposed beta-activity causing vasodilation and increased cardiac rate (epinephrine reversal). · major
- Metoclopramide · Phenothiazines may potentiate the extrapyramidal effects of metoclopramide.
- Monoamine Oxidase Inhibitors (MAOIs) · May potentiate extrapyramidal effects.
- Opiates · May enhance the hypotensive effects of phenothiazines; dosages may need to be reduced.
- Organophosphate Agents · Phenothiazines should not be given within one month of worming with these agents as their toxic effects may be potentiated.
- CNS Depressants (e.g., opioids, barbiturates, anesthetics) · Additive central nervous system depression and sedation. · major
- Anticholinergic agents (e.g., atropine, glycopyrrolate) · Additive anticholinergic effects (tachycardia, dry mouth, ileus). · moderate
Monitorización
- Clinical efficacy (resolution of vomiting or pruritus)
- Degree of sedation
- Hydration status (if vomiting persists)
- Heart rate and rhythm
- Blood pressure (especially with IV use)
- Level of consciousness / CNS status
- Respiratory rate
Sobredosis
Overdose may result in profound sedation, severe hypotension, or acute extrapyramidal clinical signs (e.g., torticollis, tremors, excessive salivation, rigidity). * **Treatment**: Supportive care is primary. Acute extrapyramidal signs have been successfully treated with injectable **diphenhydramine** in humans. Hypotension should be treated with IV fluids; avoid epinephrine due to the risk of epinephrine reversal (use alpha-agonists like phenylephrine if pressors are required).
La referencia de fármacos de VetSheet está destinada a veterinarios colegiados como apoyo a la decisión clínica, no sustituye el juicio profesional ni la ficha técnica vigente del fabricante.