Why this drug is interesting

Ephedrine is the drug an anaesthetist reaches for when a spinal drops the blood pressure, and it has been for seventy years. It raises cardiac output and systemic vascular resistance, it does not need a central line, it comes in a ready-to-use syringe, and it does not drop the heart rate the way metaraminol or phenylephrine can. It is unglamorous and it works.

What is interesting is how loosely that routine practice is licensed. There are four ephedrine injection products on the electronic Medicines Compendium and they do not say the same thing — about the indication, about children, or about how much you may give in a day. There are also two strengths, ten times apart, both licensed for the same job, and the dose you want is 3 to 6 milligrams.

Dosing

The ready-to-use 3 mg/mL pre-filled syringe (Aguettant)

Slow intravenous injection of 3 to 6 mg (maximum 9 mg), repeated as needed every 3–4 min to a maximum of 30 mg.¹
"A lack of efficacy after 30 mg should lead to reconsideration of the choice of the therapeutic agent."¹
The dose administered for 24 hours must not exceed 150 mg.¹
Elderly: as for adults — no reduction stated
No dose adjustment for renal or hepatic impairment
"Ephedrine must be used solely by or under the supervision of the anaesthetist"¹

The 30 mg/mL ampoules (Martindale, hameln)

Up to 30 mg in increments of 3–7.5 mg, by slow intravenous injection after the development of hypotension²³
No 24-hour ceiling is given
The hameln label directs the reader to §6.6 for dilution instructions. §6.6 names compatible diluents — sodium chloride 0.9%, Ringer's lactate, glucose 5% — but does not state a target concentration or volume.³ The instruction points somewhere that does not finish the sentence.

Children

Label	Paediatric position
Aguettant 3 mg/mL pre-filled syringe¹	"generally not recommended for use in children due to insufficient data"; under 12 years "not been established. No data are available"; over 12 years, as for adults
hameln 30 mg/mL³	Same wording — not recommended; under 12 no data; over 12 as adults
Martindale 3 mg/mL ampoule⁴	Under 12 years: "Not recommended." Over 12 as adults
Martindale 30 mg/mL²	Gives a dose: "0.5 – 0.75 mg / kg body weight or 17 – 25 mg / m2 body surface"

The indication is narrower than the practice

Ephedrine is given routinely for hypotension after induction of general anaesthesia. Only one UK label licenses that.

Product	§4.1 Therapeutic indication, verbatim
Aguettant 3 mg/mL pre-filled syringe¹	"Treatment of hypotension from spinal, epidural, and general* anaesthesia."*
Martindale 30 mg/mL²	"Reversal of hypotension from spinal or epidural anaesthesia."
hameln 30 mg/mL³	"Reversal of hypotension from spinal or epidural anaesthesia."
Martindale 3 mg/mL ampoule⁴	"Reversal of hypotension from spinal or epidural anaesthesia."

The oral product is a separate matter. Ephedrine hydrochloride 15 mg and 30 mg tablets remain licensed in the UK "for the treatment or prevention of attacks of bronchospasm in asthma", at 15–60 mg three times daily, with 15 mg tds in 1–5 year olds and 30 mg tds in 6–12 year olds.⁵ No current asthma guideline recommends oral ephedrine. The indication is a historical survival.

The volatile anaesthetic problem

Ephedrine's label flags inhalational anaesthetics twice, in two different sections, and sevoflurane's label flags ephedrine twice, in two different entries. Both drugs are nonetheless used together many times a day in every UK theatre.

Ephedrine §4.4"Ephedrine should be avoided or used with caution in patients undergoing anaesthesia with cyclopropane, halothane, or other halogenated anaesthetics, as they may induce ventricular fibrillation."¹

Ephedrine §4.5Halogenated volatile anaesthetics — listed under "Combinations not recommended": "Risk of perioperative hypertensive crisis and serious ventricular arrhythmias."¹

Sevoflurane §4.5"The use of amphetamines and derivatives as well as of ephedrine and derivatives can cause preoperative hypertensive crisis. It is preferable to interrupt treatments some days before surgery."⁶

Sevoflurane §4.5 againIndirect-acting Sympathomimetics: "There is a risk of acute hypertensive episode with the concomitant use of sevoflurane and indirect-acting sympathomimetic products (amphetamines, ephedrine)."⁶

How to read these four warnings honestly

The sevoflurane entry that gives its reasoning is clearly about chronic therapy — it advises interrupting treatment days before surgery, which only makes sense for an oral drug a patient is already taking. The second sevoflurane entry and both ephedrine entries are unqualified: they do not distinguish standing oral ephedrine from a single 6 mg intravenous increment given by the anaesthetist to treat the hypotension the volatile just caused. Neither does the named class exposure: the drug that genuinely sensitised the myocardium enough to make this a famous interaction was halothane, which ephedrine's §4.4 names first and which has not been in routine UK use for decades. The labelled warning is a class statement inherited from a different agent, carried forward unchanged, and never reconciled with the fact that ephedrine's own newest licence covers hypotension from general anaesthesia.

Pharmacology

Mechanism

The SmPC states it compactly: ephedrine "is a sympathomimetic amine acting directly on the alpha and beta receptors and indirectly by increasing the release of noradrenaline by the sympathetic nerve endings", and it "stimulates the central nervous system, the cardiovascular system, the respiratory system, and the sphincters of the digestive and urinary systems".¹ Blood pressure rises through a combination of increased cardiac output and increased systemic vascular resistance, which is why heart rate is usually maintained or increased rather than reflexly slowed.

Ephedrine is also an MAO inhibitor

The label's own §5.1 says so in one line: "Ephedrine is also a monoamine oxidase (MAO) inhibitor."¹ It is a single unelaborated sentence in §5.1, and it is rarely taught. It is not the reason MAO inhibitors are contraindicated, and this page is not going to present it as such. That contraindication runs the other way: an MAO inhibitor prevents breakdown of the noradrenaline that ephedrine releases, which is why non-selective MAOIs are an absolute contraindication in §4.3 — including within 14 days of withdrawal — and why selective MAO-A inhibitors and linezolid are "not recommended", all for "paroxysmal hypertension, hyperthermia possibly fatal".¹ Treat the label's MAO-inhibitor line as an uncontextualised statement about the molecule, not as an explanation of the interaction list.

Kinetics — and why urine pH matters

Bioavailability: Complete after IV; above 90% orally
Excretion: 77% unchanged in urine
Half-life at urine pH 5: 3 hours
Half-life at urine pH 6.3: approximately 6 hours
Fraction excreted, acidic urine: 73–99% (mean 88%)
Fraction excreted, alkaline urine: 22–35% (mean 27%)

All of those figures are from §5.2 of the Aguettant SmPC.¹ The dependence of clearance on urine pH is the pharmacological fact behind ephedrine's long history in doping control, and the label warns about it directly: "Athletes should be informed that this preparation contains an active substance which might give a positive reaction in anti-doping tests."¹

Where tachyphylaxis sits

Adverse effects and overdose

Labelled frequencies (Aguettant §4.8)¹

Common — palpitations, hypertension, tachycardia; nervousness, irritability, restlessness, weakness, insomnia, headache, sweating; confusion, anxiety, depression; dyspnoea; nausea, vomiting
Rare — cardiac arrhythmia; acute urinary retention
Not known — anginal pain, reflex bradycardia, cardiac arrest, hypotension; cerebral haemorrhage; pulmonary oedema; psychotic states; angle-closure glaucoma; tremor; hypokalaemia and changes in blood glucose

Overdose

The labels describe nausea, vomiting, fever, paranoid psychosis, ventricular and supraventricular arrhythmia, hypertension, respiratory depression, convulsions and coma.¹ Management, stated identically across the injection labels:

Supraventricular tachycardia — slow intravenous labetalol 50–200 mg with ECG monitoring.¹²³⁴
Marked hypokalaemia (<2.8 mmol/L) is due to a compartmental shift of potassium, predisposes to arrhythmia, and "may be corrected by infusing potassium chloride in addition to propranolol and correcting respiratory alkalosis, when present".¹
Severe hypertension — intravenous nitrates, calcium channel blockers, sodium nitroprusside, labetalol or phentolamine, chosen on availability and the patient's state.¹
CNS stimulation — a benzodiazepine and/or a neuroleptic may be required.

Practical use

Read the strength on the ampoule before you draw up. 3 mg/mL and 30 mg/mL are both stocked in UK hospitals, both are licensed for anaesthetic hypotension, and the dose is 3–6 mg.
If it is 30 mg/mL, follow your local preparation standard — the labels will not give you one. The hameln label directs you to dilute before administration and names sodium chloride 0.9%, Ringer's lactate and glucose 5% as compatible, but states no target concentration;³ the Martindale 30 mg/mL label says nothing about dilution at all.² This page does not supply a concentration either — inventing one would be exactly the error the rest of this page is about.
Give 3–6 mg slowly, wait 3–4 minutes, repeat. The onset is not instant and stacking doses inside the interval is how patients become hypertensive and tachycardic.
Stop at 30 mg and change drug. That is what the label says to do,¹ and it is the practical form of the tachyphylaxis teaching.
Treat the cause as well as the number. Spinal-induced hypotension is largely a vasodilatation and preload problem; ephedrine is buying time while position, fluid and the block level are addressed.
Do not mix it in the line. "In the absence of compatibility studies, this medicinal product must not be mixed with other medicinal products."¹
Check for a non-selective MAO inhibitor. That is an absolute contraindication, including within 14 days of withdrawal, as are concurrent indirect or alpha sympathomimetics.¹

Choosing between the theatre pressors

Drug	Action	Heart rate	Main reservation
Ephedrine	Mixed direct α/β + indirect noradrenaline release	Maintained or increased	Tachyphylaxis; 30 mg ceiling; CNS stimulation
Metaraminol	Predominantly α, some indirect	May fall reflexly	UK/Australasia-specific drug; tachyphylaxis expected
Noradrenaline	Direct α ≫ β	May fall reflexly	Base vs tartrate is a twofold hazard; extravasation

Critical appraisal

The four UK injection labels are not interchangeable documents and nobody treats them as four. They differ on the indication, on children, and on the 24-hour ceiling. Clinicians reasonably read "the ephedrine SmPC" as a single thing; there is no such thing.
The tenfold strength difference is the real hazard and it is a supply decision, not a clinical one. No pharmacological argument requires a 30 mg/mL presentation of a drug dosed in 3 mg increments. The dilution step exists only because the ampoule does.
The volatile-anaesthetic interaction is a genuine labelled warning whose evidence belongs to halothane. It is carried forward in modern labels unchanged, is unqualified as to route and duration, and sits alongside a licence for exactly the combination it warns against. That is a labelling failure, not a clinical controversy — but it should be named as a labelling failure rather than quietly ignored.
The asthma indication should have gone. An oral sympathomimetic licensed for paediatric asthma, with contraindications stricter than those of the injection, is a live licence for a treatment no guideline recommends. Its persistence is an argument about regulatory housekeeping, not about ephedrine.
Tachyphylaxis is the received mechanism here, and received mechanisms on this site have a poor record. The depletion explanation is coherent and consistent with the label's 30 mg ceiling, but it is not a labelled statement, and this page does not claim it is.

References

1
Ephedrine Hydrochloride 3 mg/ml Solution for Injection in Pre-filled Syringe — Summary of Product Characteristics, Laboratoire Aguettant. electronic Medicines Compendium, product 5354. Sections 4.1, 4.2, 4.3, 4.4, 4.5, 4.8, 4.9, 5.1, 5.2, 6.2. Text revised 19 Dec 2022; verified 6 Oct 2026.
2
Ephedrine Hydrochloride 30 mg per 1 ml Solution for Injection — Summary of Product Characteristics, Martindale Pharma (Ethypharm Group). emc, product 3583. Sections 4.1, 4.2, 4.9. Verified 6 Oct 2026.
3
Ephedrine Hydrochloride 30 mg/ml Solution for Injection — Summary of Product Characteristics, hameln pharma ltd. emc, product 9291. Sections 4.1, 4.2, 4.9, 6.6. Verified 6 Oct 2026.
4
Ephedrine Hydrochloride Injection 3mg per ml (ampoule) — Summary of Product Characteristics, Martindale Pharma (Ethypharm Group). emc, product 3594. Sections 4.1, 4.2, 4.9. Verified 6 Oct 2026.
5
Ephedrine Hydrochloride 30mg Tablets — Summary of Product Characteristics, Wockhardt UK Ltd. emc, product 2578. Sections 4.1, 4.2, 4.3. Verified 6 Oct 2026. Companion product: Ephedrine Hydrochloride 15mg Tablets, emc 2577.
6
Sevoflurane 100% Inhalation Vapour, liquid — Summary of Product Characteristics, Piramal Critical Care Ltd. emc, product 9780. Section 4.5. Text updated 12 Aug 2026; verified 6 Oct 2026. See the sevoflurane monograph.
7
Ephedrine hydrochloride — dosing and safety monograph. BNF, NICE. bnf.nice.org.uk

Ephedrine

At a glance