# Phenylephrine: veterinary dose and monograph

> Phenylephrine is a veterinary cardiovascular drug. A PURE α1-adrenergic agonist (essentially no β activity) vasopressor — it raises blood pressure by vasoconstriction alone, causing a reflex bradycardia (unlike norepinephrine/adrenaline). Indications include anaesthesia-associated hypotension / vasodilation (bolus or CRI); increasing systemic vascular resistance where beneficial (some cardiac shunts, dynamic outflow obstruction); ophthalmic mydriasis / diagnostic testing (incl. Horner's syndrome localisation). This monograph lists 4 reference doses for dogs, cats and horses, each with route, frequency and source.

Source: https://vetmasterji.com/drugs/phenylephrine · Last reviewed: 2026-10-01

## At a glance

- **Drug class:** Cardiovascular
- **Also known as:** Neosynephrine, Phenylephrine HCl, Drosyn
- **Species with doses:** Dogs, Cats and Horses
- **Routes:** IV
- **Last reviewed:** 1 October 2026

## Indications

- Anaesthesia-associated hypotension / vasodilation (bolus or CRI)
- Increasing systemic vascular resistance where beneficial (some cardiac shunts, dynamic outflow obstruction)
- Ophthalmic mydriasis / diagnostic testing (incl. Horner's syndrome localisation)
- Topical nasal/mucosal decongestion

## Phenylephrine dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

### Dogs

*Phenylephrine doses for dogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Hypotension — CRI | 1–3 µg/kg/min | IV | CRI, titrated to BP | — |
| Hypotension — bolus | 0.01 mg/kg | IV | q15min as needed (or 0.1 mg/kg IM/SC q15min) | — |

- Hypotension — CRI: Papich 5e: 10 µg/kg IV then 3 µg/kg/min. Plumb’s p.966: 1 µg/kg/min (low) to 3 µg/kg/min (high) (Quandt; Rudloff); 2–10 µg/kg/min for profound septic vasodilation (Ko). Expect reflex bradycardia; excessive splanchnic and renal vasoconstriction possible. Correct hypovolaemia first.
- Hypotension — bolus: Papich 5e (dogs and cats): 10 µg/kg IV q15min as needed, or 0.1 mg/kg IM or SC q15min.

### Cats

*Phenylephrine doses for cats*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Hypotension — CRI | 1–3 µg/kg/min | IV | CRI, titrated to BP | — |

- Hypotension — CRI: Plumb’s p.966: 1–2 µg/kg/min raised MAP without changing cardiac output (Pascoe 2006) — useful with HCM or marked vasodilation; 1–3 µg/kg/min (Quandt; Rudloff); up to 2–10 in septic shock (Ko). Papich 5e: 10 µg/kg IV then 3 µg/kg/min. Ophthalmic phenylephrine for Horner’s localisation is a separate topical product.

### Horses

*Phenylephrine doses for horses*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Nephrosplenic entrapment (splenic contraction) | 10–20 mg in 500 mL saline IV over 10–15 min (or 20 mg in 1 L over 10 min) | IV | once, before rolling or exercise | — |

- Nephrosplenic entrapment (splenic contraction): Papich 5e — controversial because of adverse effects: fatal haemorrhage has been reported, especially in older horses. Plumb’s p.966: 5 mg IV (Enos & Keiser). ARCI Class 3.

## Contraindications

Uncorrected hypovolaemia (resuscitate first). Severe bradyarrhythmia (reflex bradycardia worsens it). Significant hypertension. Extravasation risk (reliable/central line). Narrow-angle glaucoma (ophthalmic). MAO inhibitors. Known hypersensitivity.

## Species-specific warnings

> **Caution:** Horses: Use for nephrosplenic (left dorsal displacement) entrapment is specialist — associated with a risk of fatal splenic haemorrhage, especially in older horses.

## Adverse effects

- Reflex BRADYCARDIA (the hallmark — pure vasoconstriction)
- Hypertension; excessive vasoconstriction/organ ischaemia at high doses
- Tissue necrosis if extravasated
- Ophthalmic: transient stinging, mydriasis, mild systemic pressor effect (high-strength drops)

## Drug interactions

- MAO inhibitors / tricyclics: potentiated pressor effect
- Alpha-blockers: antagonise the effect
- Other vasopressors: additive vasoconstriction
- Oxytocics/ergot: additive hypertension

## Pregnancy and lactation

Vasoconstriction can reduce uteroplacental flow — use only if maternal benefit is clear; limited data.

## Monitoring

- Blood pressure and heart rate (expect reflex bradycardia) — titrate to target
- Perfusion (avoid over-vasoconstriction); IV site (extravasation)
- Volume status (correct hypovolaemia first)

## Toxicity and overdose

Overdose causes marked hypertension, severe reflex bradycardia, and excessive vasoconstriction with peripheral/organ ischaemia. Extravasation causes local necrosis. Effects are short-lived; ophthalmic high-strength drops can cause systemic pressor effects, especially in small/neonatal patients.

Management: Reduce/stop the infusion. For severe hypertension/vasoconstriction, an alpha-blocker (phentolamine) can be used, including local infiltration at an extravasation site. Manage reflex bradycardia if problematic (usually resolves as BP normalises); continuous BP/ECG monitoring.

## Clinical pearls

- The "pure vasoconstrictor" — raises blood pressure by α1 alone, with a characteristic REFLEX BRADYCARDIA (and no cardiac stimulation), useful when you want SVR up without tachycardia
- Handy for anaesthetic hypotension (bolus or CRI) and as an ophthalmic mydriatic / Horner's-syndrome localiser
- Correct hypovolaemia first, watch for extravasation necrosis (phentolamine is the rescue), and anticipate the reflex bradycardia
- In horses it is occasionally used to contract the spleen for nephrosplenic entrapment — a specialist manoeuvre with haemorrhage risk

## Mechanism of action

A selective α1-adrenergic agonist that constricts arterial and venous smooth muscle, raising systemic vascular resistance and blood pressure with essentially NO direct cardiac (β) stimulation — so the rise in pressure triggers a baroreceptor-mediated REFLEX bradycardia. In the eye it contracts the iris dilator muscle (mydriasis) and conjunctival vessels; on mucosa it produces decongestant vasoconstriction.

## Formulations and products

- Injection 10 mg/mL (dilute for IV)
- Ophthalmic solution 2.5 / 10 % (mydriatic/diagnostic)
- Phenylephrine injection / Drosyn eye drops — 10 mg/mL injection; 2.5 / 10 % eye drops, Injection / ophthalmic (human-label)

## Storage

Injection: 15–30 °C, protect from light (discard if discoloured). Ophthalmic: per label. Dilute for IV infusion.

## Before you use this dose

> **Caution:** Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

## Other cardiovascular drugs

- [Metoprolol](https://vetmasterji.com/drugs/metoprolol)
- [Mexiletine](https://vetmasterji.com/drugs/mexiletine)
- [Nitroglycerin (Glyceryl Trinitrate)](https://vetmasterji.com/drugs/nitroglycerin)
- [Norepinephrine (Noradrenaline)](https://vetmasterji.com/drugs/norepinephrine)
- [Pimobendan](https://vetmasterji.com/drugs/pimobendan)
- [Procainamide](https://vetmasterji.com/drugs/procainamide)
- [Propranolol](https://vetmasterji.com/drugs/propranolol)
- [Quinidine](https://vetmasterji.com/drugs/quinidine)

## Frequently asked questions

### What is the dose of Phenylephrine for dogs?

Reference doses for Phenylephrine in dogs: 1–3 µg/kg/min IV CRI, titrated to BP (Hypotension — CRI); 0.01 mg/kg IV q15min as needed (or 0.1 mg/kg IM/SC q15min) (Hypotension — bolus). Confirm against the label and the patient before use.

### What is the dose of Phenylephrine for cats?

Reference doses for Phenylephrine in cats: 1–3 µg/kg/min IV CRI, titrated to BP (Hypotension — CRI). Confirm against the label and the patient before use.

### What is the dose of Phenylephrine for horses?

Reference doses for Phenylephrine in horses: 10–20 mg in 500 mL saline IV over 10–15 min (or 20 mg in 1 L over 10 min) IV once, before rolling or exercise (Nephrosplenic entrapment (splenic contraction)). Confirm against the label and the patient before use.

### When should Phenylephrine not be used?

Uncorrected hypovolaemia (resuscitate first). Severe bradyarrhythmia (reflex bradycardia worsens it). Significant hypertension. Extravasation risk (reliable/central line). Narrow-angle glaucoma (ophthalmic). MAO inhibitors. Known hypersensitivity.

### What are the side effects of Phenylephrine in animals?

Reflex BRADYCARDIA (the hallmark — pure vasoconstriction); Hypertension; excessive vasoconstriction/organ ischaemia at high doses; Tissue necrosis if extravasated; Ophthalmic: transient stinging, mydriasis, mild systemic pressor effect (high-strength drops).

## Sources

- Plumb's Veterinary Drug Handbook
- Merck Veterinary Manual, 11th edition
- Papich, Papich Handbook of Veterinary Drugs, 5th edition

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.