Pharmacology · Vasopressors and inotropes
Phenylephrine !
A pure alpha-1 agonist that tightens vessels. Given intravenously for low blood pressure during anaesthesia; it typically slows the heart by reflex.
Updated 2026-11-01
No structure of phenylephrine bound to its receptor is available; the figure shows noradrenaline in the same binding site of the human alpha-1A adrenergic receptor.
Mechanism of action
Phenylephrine selectively stimulates alpha-1 receptors, constricting arterioles and veins in most vascular beds, including renal, splanchnic and pulmonary arteries. Blood pressure rises; the baroreflex increases vagal tone and slows the heart, so cardiac output can fall. Locally it also dilates the pupil and shrinks nasal mucosa.
- Phenylephrine stimulates vascular alpha-1 receptors
- Arterioles and veins constrict; blood pressure rises
- The baroreflex slows the heart (reflex bradycardia)
Pharmacokinetics
- Distribution
- Large volume of distribution into organs and tissues
- Metabolism
- Mainly by MAO and sulfotransferase; it is not a catechol, so COMT does not inactivate it
- Half-life
- Effective half-life about 5 minutes after infusion (label)
Indications
- Clinically important hypotension from vasodilation during anaesthesia
- Other forms: nasal decongestant, short-acting mydriatic for eye examination
Adverse effects
Common
- Nausea, vomiting and headache
- Reflex bradycardia
- Hypertension
- Reduced cardiac output
Contraindications
- None listed on the label; caution in severe heart disease
Key interactions
- MAO inhibitors, tricyclic antidepressants, oxytocic drugs, atropine and ergot alkaloids
- Stronger pressor effect; with oxytocics, risk of haemorrhagic stroke
- Alpha blockers, PDE-5 inhibitors, calcium channel blockers and ACE inhibitors
- Weaker pressor effect
What is monitored
- Blood pressure and heart rate continuously
- Peripheral perfusion
- Kidney function in septic shock
- Infusion site
Administration and safety
- Intravenous bolus or infusion after dilution, by trained staff with continuous monitoring.
- Check the infusion site often for free flow; extravasation can cause necrosis.
- Concentrations differ between products: check the label before preparing it.
- In end-stage kidney disease patients may respond more strongly.
Nursing care
- Check heart rate as well as blood pressure: a slow heart is an expected reflex but can be severe.
- Watch the colour and temperature of the limbs and the infusion site.
- Report chest pain, breathlessness or falling urine output.
Pregnancy and breastfeeding
Studies during caesarean section have not shown a risk of birth defects, and it does not appear to affect fetal heart rate much at usual doses.
Test yourself
Think of the answer, then tap the question to see it.
Why does phenylephrine often slow the heart?
The rise in blood pressure from pure alpha-1 vasoconstriction activates the baroreflex, increasing vagal tone (reflex bradycardia).
In what clinical setting is intravenous phenylephrine mainly used?
Hypotension from vasodilation during anaesthesia, for example after spinal anaesthesia.
Which drugs weaken the pressor effect of phenylephrine?
Alpha blockers, PDE-5 inhibitors, calcium channel blockers, ACE inhibitors and central sympatholytics, among others.
Related drugs
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - PHENYLEPHRINE HYDROCHLORIDE injection (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), cap. 9 (McGraw Hill, 2025).
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Representative model, for educational purposes only.