Phenylephrine
Phenylephrine Hydrochloride
Also known as: Neo-Synephrine · AH-chew D · Little Colds Decongestant · Lusonal · Nasop · Sudogest PE · Sudafed PE · Pedia Care Children's Decongestant · Triaminic Thin Strips Cold · Minims Phenylephrine · fenilefrina · phenylephrinum · m-synephrine · Phenylephrine hydrochloride
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| As a CRI to increase peripheral vascular resistance and mean arterial blood pressure | Low dose is 1 microgram/kg/min; high dose is 3 micrograms/kg/min | IV | CRI | — | 🌎 NA |
| As a constant rate infusion for profound vasodilation due to septic shock | 2-10 micrograms/kg/minute | IV | CRI | — | 🌎 NA |
| As a vasopressor in catastrophic stages of hypovolemic shock | 1-3 micrograms/kg/min | IV | CRI | — | 🌎 NA |
| Diagnosis of Horner's syndrome (denervation hypersensitivity) | 1% or 10% solution topically to both eyes | topical | once | single dose | 🌍 EU |
| Hypotension secondary to drugs or vascular failure | Used in conjunction with fluid therapy (exact dose not specified in monograph) | IV | CRI or as directed | Until blood pressure stabilizes | 🌍 EU |
- As a CRI to increase peripheral vascular resistance and mean arterial blood pressure: May see reflex bradycardia, and vasoconstriction can lead to excessive decreases in blood flow to liver, GI tract, and kidneys, although coronary blood flow is increased.
- Diagnosis of Horner's syndrome (denervation hypersensitivity): Time to dilation indicates lesion location: <20 mins = 3rd-order HS; 20-45 mins = 2nd-order HS; 60-90 mins = 1st-order HS or normal. 10% solution causes mydriasis in 5-8 mins in post-ganglionic lesions.
- Hypotension secondary to drugs or vascular failure: Use with caution; raises BP at the expense of vital organ perfusion.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| To increase blood pressure by vasoconstriction in pronounced systemic vasodilation or when increasing myocardial contractility is disadvantageous (e.g., hypertrophic cardiomyopathy) | 1-2 micrograms/kg/minute | IV | CRI | — | 🌎 NA |
| As a constant rate infusion for profound vasodilation due to septic shock | 2-10 micrograms/kg/minute | IV | CRI | — | 🌎 NA |
| As a vasopressor in catastrophic stages of hypovolemic shock | 1-3 micrograms/kg/min | IV | CRI | — | 🌎 NA |
| As a CRI to increase peripheral vascular resistance and mean arterial blood pressure | Low dose is 1 microgram/kg/min; high dose is 3 micrograms/kg/min | IV | CRI | — | 🌎 NA |
| Diagnosis of Horner's syndrome (denervation hypersensitivity) | 1% solution topically to both eyes | topical | once | single dose | 🌍 EU |
- To increase blood pressure by vasoconstriction in pronounced systemic vasodilation or when increasing myocardial contractility is disadvantageous (e.g., hypertrophic cardiomyopathy): Infusions of 1 microgram/kg/min significantly increased mean arterial pressure without a change in cardiac output. At 2 micrograms/kg/min, cardiac index also was increased with an increase in stroke volume index.
- As a CRI to increase peripheral vascular resistance and mean arterial blood pressure: May see reflex bradycardia, and vasoconstriction can lead to excessive decreases in blood flow to liver, GI tract, and kidneys, although coronary blood flow is increased.
- Diagnosis of Horner's syndrome (denervation hypersensitivity): Use lower concentrate solutions in cats to avoid systemic hypertension.
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| General use | 5 mg | IV | Single dose | — | 🌎 NA |
- General use: ARCI UCGFS Class 3 Drug
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Phenylephrine is a potent, direct-acting synthetic sympathomimetic amine primarily utilized in veterinary medicine for its profound vasoconstrictive properties.
Key clinical applications include:
- Hypotension and Shock: Used parenterally to elevate blood pressure (after adequate volume replacement) without causing overt cardiostimulation.
- Drug-Induced Hypotension: Highly effective for hypotensive crises secondary to drug overdoses or idiosyncratic reactions (e.g., phenothiazines, adrenergic blocking agents).
- Ophthalmic Use: Applied topically for diagnostic eye examinations, reducing posterior synechiae formation, and relieving pain associated with complicated uveitis.
- Nasal Decongestion: Applied intranasally to reduce mucosal congestion.
Clinical Pearl: Because phenylephrine lacks significant beta-adrenergic activity at normal doses, it is particularly useful when an increase in blood pressure is needed but an increase in heart rate or myocardial work is contraindicated (e.g., in patients with hypertrophic cardiomyopathy).
Mechanism of action
Phenylephrine is a highly selective alpha-1 adrenergic receptor agonist.
- Vascular Smooth Muscle: Binds to post-synaptic alpha-1 receptors → activates Gq-proteins → stimulates Phospholipase C (PLC) → cleaves PIP2 into Inositol Triphosphate (IP3) and Diacylglycerol (DAG) → IP3 triggers the release of intracellular calcium from the sarcoplasmic reticulum → profound vasoconstriction.
- Cardiovascular Effects: Intravenous administration causes peripheral vasoconstriction, leading to significant increases in both diastolic and systolic blood pressures. This sudden increase in blood pressure often triggers a baroreceptor-mediated reflex bradycardia (which can be blocked by anticholinergics like atropine).
- Organ Perfusion: Constricts most vascular beds (renal, splanchnic, pulmonary, cutaneous), though coronary blood flow is typically increased due to elevated aortic diastolic pressure.
Safety & warnings
Contraindications
- Severe hypertension
- Ventricular tachycardia
- Hypersensitivity to phenylephrine
- Do not apply topically once ophthalmic surgery has started (to avoid direct arterial absorption)
Adverse effects
- Reflex bradycardia
- CNS effects (excitement, restlessness, headache)
- Arrhythmias (rare)
- Severe extravasation injuries (tissue necrosis and sloughing)
- Hypertension
- Tachycardia
- Tissue necrosis and sloughing (if IV extravasation occurs)
Precautions
WARNING: Extravasation injuries with phenylephrine can be very serious, leading to necrosis and sloughing of surrounding tissue. IV sites must be routinely monitored. If extravasation occurs, infiltrate the ischemic area with 5-10 mg phentolamine in 10-15 mL of normal saline using a fine needle.
- Extreme Caution: Use carefully in geriatric patients, and those with hyperthyroidism, bradycardia, partial heart block, or other heart disease.
- Not a Substitute for Fluids: Phenylephrine is not a replacement for adequate volume therapy in patients with shock; hypovolemia must be corrected first.
- Pregnancy: FDA Category C. Can cause contraction of the pregnant uterus and constriction of uterine blood vessels. Use with caution.
Drug interactions
Higher dosages of phenylephrine may be required to attain a pressor effect if these agents have been used prior to therapy.
Potentially may induce cardiac arrhythmias when used with halothane anesthesia.
Block the reflex bradycardia caused by phenylephrine.
The cardiostimulatory effects of phenylephrine (seen at high doses) can be blocked.
Use with phenylephrine may cause increased myocardium sensitization.
Should not be used with phenylephrine because of a pronounced pressor effect.
When used concurrently with oxytocic agents, pressor effects may be enhanced.
Tachycardia and serious arrhythmias are possible.
Increased risk of cardiac arrhythmias
Enhanced pressor effects leading to severe hypertension
Monitoring
- Cardiac rate and rhythm
- Blood gases (if possible)
- IV catheter site (for signs of extravasation)
- Blood pressure (systemic use or high-concentration topical use)
- Heart rate and rhythm (ECG)
- Pupillary response and time to dilation (for Horner's syndrome testing)
Pharmacokinetics
Half-life
Absorption
After oral administration, phenylephrine is rapidly metabolized in the GI tract and cardiovascular effects are generally unattainable via this route. Following IV administration, pressor effects begin almost immediately and persist for up to 20 minutes. IM onset is 10-15 minutes, lasting approximately one hour.
Distribution
It is unknown if phenylephrine is excreted into milk.
Metabolism
Metabolized by the liver.
Elimination
The effects of the drug are terminated by uptake into tissues.
Overdose
The margin of safety with phenylephrine overdose is fairly wide, especially after oral administration.
Clinical Signs:
- Common findings in dogs include vomiting, lethargy, depression, hyperactivity, and tachycardia.
- Severe overdosage can cause hypertension, seizures, paresthesias, ventricular extrasystoles, and cerebral hemorrhage.
Treatment:
- Vomiting is commonly seen and may self-decontaminate oral exposures.
- Cardiovascular changes often respond well to IV fluids.
- Beta-blockers or nitroprusside may be indicated when signs (tachycardia, severe hypertension) are refractory to fluids.
Available products
Formulations
- Injection
- Oral tablets
- Oral solution/liquid
- Oral strips
- Ophthalmic drops
- Intranasal sprays
- Injectable: 1% (10 mg/ml) solution
- Ophthalmic: 2.5% solution (single-dose vials)
- Ophthalmic: 10% solution (single-dose vials)
Veterinary
- Oral combination products marketed as 'cough' syrups containing phenylephrine, pyrilamine, guaifenesin, sodium citrate, and sometimes ammonium chloride.
- Phenylephrine 1% injectable solution
- Phenylephrine 2.5% ophthalmic solution
- Phenylephrine 10% ophthalmic solution
Human-labeled
- Phenylephrine HCl Oral Tablets: 10 mg (regular & chewable)
- Phenylephrine HCl Oral Solution/Liquid: 2.5 mg/mL (concentrate), 2.5 mg/5mL & 7.5 mg/5 mL
- Phenylephrine HCl Oral Strips: 2.5 mg
- Phenylephrine HCl Injection: 1% (10 mg/mL) in 1 mL & 5 mL vials and 1 mL Uni-Nest amps
- Ophthalmic and intranasal dosage forms
- Minims Phenylephrine Hydrochloride 2.5% and 10% eye drops
Regulatory status
POM (Prescription Only Medicine)
POM-V or POM depending on specific formulation.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
The injectable product should be stored protected from light. Do not use solutions if they are brown or contain a precipitate. Oxidation of the drug can occur without a color change. Commercially available ampules have air replaced with nitrogen and a sulfite added to protect against oxidation.
Client information
- Hospital Use: Injectable phenylephrine is a potent cardiovascular drug that should only be used by veterinary professionals in a clinical setting where continuous blood pressure and ECG monitoring are available.
- Oral Products: Over-the-counter human cold medications containing phenylephrine should never be given to pets without explicit veterinary instruction, as they often contain other toxic ingredients (like acetaminophen or decongestants) and can cause vomiting, hyperactivity, or heart rate changes.
- Accidental Ingestion: If your pet accidentally ingests a product containing phenylephrine, contact your veterinarian or an animal poison control center immediately.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.