Alpha-1 Adrenergic Agonist (Pure Vasoconstrictor) Pregnancy: Use of injectable phenylephrine is possible during pregnancy in accordance with the indications, but animal studies are insufficient with respect to reproductive toxicity and teratogenicity, and administration in late pregnancy or labour may potentially cause fetal hypoxia and bradycardia. Combination with some oxytocic agents can cause severe hypertension. Small quantities are excreted into breast milk; after a single bolus administration during childbirth, breast-feeding is possible.

Phenylephrine

Brand names: Metaraminol (different drug, sometimes confused)

Phenylephrine is a selective alpha-1 adrenergic agonist vasopressor used to treat hypotension, particularly during anaesthesia and spinal/epidural-induced hypotension.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Intravenous bolus injection: 50 to 100 micrograms, which can be repeated until the desired effect is attained. Continuous infusion: initial dose 25 to 50 micrograms/min

Route: Parenteral — intravenous bolus injection or continuous intravenous infusion

Frequency: Bolus repeated until the desired effect is attained; infusion increased or decreased to maintain the systolic blood pressure close to the normal value

Max: One bolus dose should not exceed 100 micrograms. By infusion, doses between 25 and 100 micrograms/min have been assessed to be effective.

Source quote (eMC §4.2): 'Intravenous bolus injection: Normal dose is 50 to 100 micrograms, which can be repeated until the desired effect is attained. One bolus dose should not exceed 100 micrograms. Continuous infusion: Initial dose is 25 to 50 micrograms/min.' UNITS: doses are stated in micrograms (bolus) and micrograms/min (infusion) — the SPC does not express phenylephrine dosing per kilogram. The fetched product is 'Phenylephrine 0.1 mg/ml, solution for injection'; the SPC states phenylephrine 50 micrograms/ml and 100 micrograms/ml solutions for injection should only be administered by healthcare professionals with appropriate training and relevant experience. Arterial blood pressure should be monitored during treatment. HEPATIC: higher doses of phenylephrine may be needed in patients with cirrhosis of the liver. ELDERLY: treatment of the elderly should be carried out with care; risk of phenylephrine toxicity is increased in elderly patients. Particular attention should be paid to avoid extravasation, since this may cause tissue necrosis. In patients with reduced cardiac output or coronary vascular disease, vital organ functions should be closely monitored and dose reduction should be considered when systemic blood pressure is near the lower end of the target range. PAEDIATRIC: the safety and efficacy of phenylephrine in children have not been established and no data are available — verify any paediatric use against a children's formulary. NOTE: the openFDA record fetched for this id was an oral OTC cold-and-flu combination product ('Daytime Cold and Flu', 30 mL every 4 hours) and is NOT applicable to parenteral phenylephrine; it has been disregarded.

Dose adjustments

Renal

eMC §4.2: lower doses of phenylephrine may be needed in patients with impaired renal function.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients
Severe hypertension or peripheral vascular disease, due to the risk of ischaemic gangrene or vascular thrombosis
Combination with non-selective monoamine oxidase inhibitors (MAOIs), or within 2 weeks of their withdrawal, due to the risk of paroxysmal hypertension and possibly fatal hyperthermia
Severe hyperthyroidism

Side effects

Reflex bradycardia — the most commonly reported cardiovascular adverse event; also tachycardia, palpitations, arrhythmia, angina pectoris and myocardial ischaemia
Hypertension and hypertensive episodes (more frequent with high doses); cerebral haemorrhage, hypertensive crisis
Nausea and vomiting
Skin necrosis with extravasation; sweating, pallor or skin blanching, piloerection
Headache, nervousness, insomnia, paraesthesia, tremor; anxiety, excitability, agitation, confusion, psychotic states
Dyspnoea and pulmonary oedema; mydriasis and aggravation of pre-existing angle-closure glaucoma

Interactions

Non-selective MAOIs (iproniazid, nialamide) — CONTRAINDICATED: paroxysmal hypertension and possibly fatal hyperthermia; the interaction is still possible 15 days after discontinuation of the MAOI
Dopaminergic ergot alkaloids (bromocriptine, cabergoline, lisuride, pergolide) — inadvisable: risk of vasoconstriction and/or hypertensive crisis
Vasoconstrictor ergot alkaloids (dihydroergotamine, ergotamine, methylergometrine, methysergide) — inadvisable: risk of vasoconstriction and/or hypertensive crisis
Tricyclic antidepressants (e.g. imipramine) and noradrenergic-serotoninergic antidepressants (milnacipran, venlafaxine) — paroxysmal hypertension with possibility of arrhythmias
Selective type A MAOIs (moclobemide, toloxatone) and linezolid — risk of vasoconstriction and/or hypertensive crisis
Some oxytocic agents — the combination can cause severe hypertension (eMC §4.6)

Clinical monograph

How it works

It stimulates alpha-1 adrenoceptors on vascular smooth muscle to produce peripheral vasoconstriction, raising systemic vascular resistance and blood pressure.

Prescribing in practice

The rise in blood pressure can trigger a reflex bradycardia, so heart rate must be watched and the drug used with caution where bradycardia or fixed cardiac output is a concern.
It is preferred where vasoconstriction without inotropy is desired, but extravasation of concentrated solutions can cause local ischaemia and tissue necrosis.
Use with caution in patients with hypertension, severe coronary disease or peripheral vascular disease, and account for exaggerated responses with monoamine oxidase inhibitors.

Monitoring

Continuous blood pressure and heart rate monitoring is required, ideally with the infusion titrated to a defined haemodynamic target.

Counselling the patient

This is given by trained staff with close blood pressure monitoring.
Report any pain or skin changes at the infusion site promptly.

Evidence & guidelines

Its use as a vasopressor, including for maternal hypotension during spinal anaesthesia for caesarean section, is well established in anaesthetic practice.

Reference: Ngan Kee (Anesthesiology 2009); OAA/AAGBI Obstetric Anaesthesia Guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators