Phenylephrine | Medication Reference

Bedside Snapshot

Drug Class: Selective α1-adrenergic agonist; vasopressor; decongestant
Mechanism: Pure α1 agonist causing arterial and venous vasoconstriction with minimal direct β1 activity at usual doses; increases systemic vascular resistance and blood pressure, often with reflex bradycardia
Core ED/ICU/OR Uses: Treatment of hypotension (especially vasodilatory or anesthetic-induced) in patients with relatively preserved cardiac output; peri-intubation hypotension; second-line or adjunct vasopressor in shock when tachyarrhythmias limit catecholamines with β activity
Typical Adult IV Bolus: 50–200 mcg IV bolus (commonly 100 mcg) over 30–60 seconds for peri-intubation or anesthetic-induced hypotension; may repeat every 1–2 minutes as needed while starting or adjusting vasopressor infusion
Typical Adult IV Infusion: Start 0.5–1 mcg/kg/min (or ~20–40 mcg/min fixed-dose) and titrate up by 0.1–0.5 mcg/kg/min every few minutes to maintain target MAP (e.g., ≥65 mmHg); common dose range 0.5–5 mcg/kg/min; some protocols allow up to ~8–9 mcg/kg/min in refractory shock
Pediatric IV Dosing: Bolus 5–20 mcg/kg (max 500 mcg) or 0.1–0.5 mcg/kg/min infusion, titrated carefully with invasive monitoring (specialist-directed; institutional protocols vary)
Onset: Rapid after IV bolus (within ~1 minute) with short clinical duration of a few minutes; continuous infusion required for sustained effect
Half-Life: Distribution half-life is minutes; elimination half-life roughly 2–3 hours
vs Norepinephrine: Phenylephrine has no direct β1 inotropy and can reduce stroke volume and cardiac output in patients who are preload-sensitive or have LV dysfunction; best suited for vasodilatory hypotension in patients with relatively intact cardiac function or when tachycardia is a major concern
Non-IV Uses: Nasal spray/drops for congestion; ophthalmic drops for mydriasis without cycloplegia; (less favored) oral decongestant
Key Risk: Can decrease heart rate and stroke volume due to reflex bradycardia and increased afterload; may worsen cardiac output in patients with impaired LV function

Brand & Generic Names

Generic Name: Phenylephrine hydrochloride
Brand Names: Neo-Synephrine (historical IV brand), Vazculep, various generics for IV use; multiple nasal/ophthalmic brands for topical decongestant use

Medication Class

Selective α1-adrenergic agonist; vasopressor; decongestant

Pharmacology

Mechanism of Action:

Phenylephrine is a selective α1-adrenergic receptor agonist with negligible direct β1 or β2 agonism at usual clinical doses
α1 stimulation in vascular smooth muscle causes vasoconstriction of arterioles and venous capacitance vessels, increasing systemic vascular resistance (SVR), venous return, and arterial blood pressure
The resulting increase in blood pressure activates baroreceptors, triggering reflex vagal outflow and often producing reflex bradycardia; heart rate may decrease even as blood pressure rises
Because it lacks direct β1 inotropic activity, phenylephrine does not increase myocardial contractility; in some patients, especially with impaired LV function or fixed stroke volume, the increased afterload and reflex bradycardia may reduce cardiac output
In nasal mucosa and conjunctival vessels, α1-mediated vasoconstriction reduces edema and congestion, which underlies phenylephrine's decongestant and mydriatic uses

Pharmacokinetics (IV focus):

Routes: IV bolus and continuous infusion for vasopressor use; also IM/SC (rarely used for shock), and topical/nasal/ophthalmic for decongestion and mydriasis
Onset (IV): Almost immediate (within ~1 minute) after bolus or change in infusion rate; peak effect is rapid
Duration (IV bolus): Clinical effect typically lasts 5–10 minutes, depending on dose and patient factors
Distribution: Initial distribution phase is rapid; apparent volume of distribution is modest (~0.4–0.9 L/kg), with limited CNS penetration
Metabolism: Primarily hepatic (via monoamine oxidase) and by sulfation in the intestinal wall; phenylephrine is also taken up and metabolized in other tissues
Elimination: Metabolites are excreted in urine; elimination half-life is typically ~2–3 hours, but hemodynamic effects in critically ill patients are governed more by receptor responsiveness and continuous infusion than by plasma half-life
Special populations: Renal and hepatic dysfunction can prolong exposure and increase sensitivity; critically ill patients may have altered receptor sensitivity and variable response

Indications

Treatment of hypotension associated with vasodilation (e.g., anesthetic-induced hypotension, neuraxial anesthesia) when cardiac output is relatively preserved
Adjunct vasopressor in septic shock or other vasodilatory shock when tachyarrhythmias, ischemia, or other factors limit the use of norepinephrine or epinephrine
Peri-intubation hypotension prophylaxis or treatment (push-dose vasopressor) in patients where increased heart rate is undesirable (e.g., tachycardia, aortic dissection, severe aortic stenosis)
Temporary support of blood pressure in neurocritical care (e.g., maintaining cerebral perfusion pressure in TBI or vasospasm) when a pure α agonist is desired
Topical nasal decongestant and ophthalmic mydriatic agent for ENT and ophthalmologic procedures (non-ICU topical uses)

Dosing & Administration

Available Forms (IV focus):

IV injection vials/ampoules: Commonly 10 mg/mL phenylephrine hydrochloride for dilution before IV administration
Premixed IV bags/syringes: Phenylephrine 40 mcg/mL or 80 mcg/mL for bolus use and 100–400 mcg/mL for infusion, depending on institutional standards
Nasal spray/drops: Typically 0.25–1% phenylephrine solutions for short-term decongestant use
Ophthalmic drops: 2.5% and 10% solutions for mydriasis; 10% concentration has greater systemic risk and is generally avoided in infants and high-risk adults

Concentration Verification: Always verify concentration and institutional preparation standards before drawing up phenylephrine; dosing errors from misdilution can be catastrophic.

Standard Dosing – IV Phenylephrine (Always Follow Local Protocols and Shock/Anesthesia Guidelines):

Indication / Population	Initial Dose	Repeat / Titration	Typical Range / Maximum
Adult peri-intubation or anesthetic-induced hypotension (IV bolus)	50–200 mcg IV bolus (commonly 100 mcg) over 30–60 seconds	May repeat every 1–2 min as needed while setting up/adjusting infusion	Total bolus dose guided by BP response; avoid repeated high doses without invasive monitoring
Adult continuous IV infusion – vasodilatory shock	0.5–1 mcg/kg/min or 20–40 mcg/min fixed starting rate	Titrate by 0.1–0.5 mcg/kg/min (or 10–20 mcg/min) every few minutes to MAP goal	Common range 0.5–5 mcg/kg/min; some protocols allow up to ~8–9 mcg/kg/min
Pediatric hypotension – IV bolus (specialist use)	5–20 mcg/kg (max 500 mcg)	Give as slow IV push; may repeat per protocol with close monitoring	Weight-based; use institutional pediatric shock guidelines
Pediatric continuous IV infusion – shock	0.1–0.5 mcg/kg/min starting rate	Titrate to BP/perfusion goals	Typical upper range ~3–5 mcg/kg/min depending on protocol

Additional Dosing Notes:

Prefer central venous access for moderate- to high-dose infusions; peripheral lines can be used short term in emergent situations with frequent limb checks and low concentrations
Avoid rapid undiluted boluses of concentrated phenylephrine (e.g., directly from 10 mg/mL vial) – always dilute to a safe working concentration (e.g., 100 mcg/mL) before IV bolus administration
Because phenylephrine can decrease heart rate and stroke volume, it may not be ideal as sole vasopressor in patients with poor LV function or fixed low cardiac output; norepinephrine or inodilators may be preferable
In neurocritical care, tighter BP targets and concerns about intracranial pressure influence titration; follow disease-specific protocols (e.g., SAH, TBI)
In pregnancy (e.g., spinal-induced hypotension during C-section), phenylephrine infusions or small boluses are favored over ephedrine in many modern obstetric anesthesia protocols due to lower fetal acidosis risk

Contraindications

Absolute or Major Contraindications:

Known hypersensitivity to phenylephrine or formulation components
Severe hypertension or severe peripheral vascular disease where additional vasoconstriction could precipitate ischemia (context-dependent)
Severe ventricular dysfunction or cardiogenic shock with low output state where increasing afterload is likely to worsen hemodynamics (relative; specialist decision)

Major Precautions:

Pre-existing bradycardia, heart block, or sick sinus syndrome: Reflex bradycardia can be pronounced; consider vasopressors with β1 activity instead
Right heart failure or pulmonary hypertension: Increased afterload and pulmonary vascular resistance may worsen RV function
Peripheral arterial disease, Raynaud phenomenon, or other vasospastic disorders: Intense vasoconstriction can precipitate ischemia of extremities or organs
Elderly patients and those with significant coronary or cerebrovascular disease: Higher risk for ischemia from excessive vasoconstriction; titrate cautiously
Concomitant MAO inhibitors, tricyclic antidepressants, or other sympathomimetics: May potentiate pressor effects; lower starting doses and careful titration are advised

Adverse Effects

Common (dose-related):

Headache, anxiety, and restlessness
Reflex bradycardia or palpitations
Hypertension (especially systolic) and gooseflesh/cold extremities
Nausea or vomiting
Local irritation at IV site

Serious:

Severe hypertension: Risk of intracranial hemorrhage, aortic dissection, or myocardial ischemia
Significant bradyarrhythmias: Including sinus bradycardia, junctional rhythms, or AV block
Decreased cardiac output: Particularly in patients with impaired LV function or severe aortic stenosis
Peripheral or visceral ischemia: Ischemia of digits, gut, kidneys from intense vasoconstriction at high doses or prolonged infusions
Extravasation: Causing local tissue ischemia and necrosis; may require local infiltration with phentolamine if recognized early

Special Populations

Renal impairment: Renal dysfunction can prolong exposure and increase sensitivity; use lower starting doses and titrate carefully with close monitoring
Hepatic impairment: Hepatic dysfunction can reduce metabolism and prolong effects; use lower starting doses and monitor hemodynamics closely
Older adults: Increased risk for ischemia, hypertension, and bradyarrhythmias; start with lower doses and titrate cautiously
Pregnancy: Phenylephrine is commonly used for spinal-induced hypotension during cesarean section and is generally preferred over ephedrine; use with careful monitoring of maternal and fetal status
Pediatrics: Weight-based dosing with specialist guidance; institutional protocols vary; use invasive monitoring when available

Monitoring Parameters (ED/ICU/OR)

Continuous ECG monitoring: For rate and rhythm; watch for bradycardia, new AV block, or ischemic changes
Frequent blood pressure monitoring: q1–2 min during titration; arterial line recommended for continuous BP measurement in shock or high-dose infusions
Assessment of perfusion: Mental status, capillary refill, skin temperature/color, urine output, lactate trends, and organ function
Inspection of infusion site(s): For signs of extravasation or local ischemia when using peripheral lines
Ongoing need reassessment: In long infusions, evaluate the ongoing need for phenylephrine versus transition to other agents as shock physiology evolves

Clinical Pearls

"Pure squeeze" drug concept: Phenylephrine is best thought of as a "pure squeeze" drug—great when vasodilation and relative tachycardia are the main problems, less ideal when pump failure or low stroke volume is dominant.

Push-dose vasopressor preparation: For peri-intubation use, pre-diluted phenylephrine (e.g., 100 mcg/mL) in labeled syringes reduces dosing errors and allows precise push-dose titration.

MAP improvement vs perfusion: If MAP improves but lactate remains high and signs of poor perfusion persist, consider that excessive vasoconstriction may be masking low cardiac output—reassess the need for phenylephrine versus adding or switching to an inotrope/other vasopressor.

Role in septic shock: In septic shock, norepinephrine remains first-line; phenylephrine is usually an adjunct when high-dose norepinephrine causes problematic tachycardia or arrhythmias, or when β-blockade is being used to control heart rate.

Documentation importance: Tight documentation of dose, concentration, titration steps, and responses is important for code reviews and quality improvement, given phenylephrine's narrow therapeutic window in some patients.

Reflex bradycardia mechanism: The reflex bradycardia with phenylephrine is mediated by baroreceptor activation from increased blood pressure. In patients with baseline bradycardia or on beta-blockers, this effect may be exaggerated and can compromise cardiac output significantly.

Neurocritical care application: In neurocritical care patients (TBI, SAH), phenylephrine can maintain cerebral perfusion pressure without the tachycardia that might increase myocardial oxygen demand, but close monitoring of cardiac output is essential.

Obstetric anesthesia preference: Modern obstetric anesthesia protocols often prefer phenylephrine over ephedrine for treating spinal-induced hypotension during cesarean section due to lower risk of fetal acidosis and better maternal hemodynamic stability.

References

1. Dawkins, T., & colleagues. (2023). Phenylephrine. In StatPearls [Internet]. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK534801/
2. DrugBank Online. (n.d.). Phenylephrine (DB00388). DrugBank. https://go.drugbank.com/drugs/DB00388
3. Drugs.com. (2024, July 18). Phenylephrine dosage guide with max dose, adjustments. Drugs.com. https://www.drugs.com/dosage/phenylephrine.html
4. Khanna, A. K., & colleagues. (2019). Vasopressor choice for hypotension in the intensive care unit. Current Opinion in Critical Care, 25(3), 254–260. https://doi.org/10.1097/MCC.0000000000000614
5. Medscape. (2024). Neo-Synephrine (phenylephrine injection) dosing, indications, interactions, adverse effects, and more. Medscape Reference. https://reference.medscape.com/drug/neo-synephrine-phenylephrine-injection-343100