Bronchodilator · Beta-2 agonist · Tocolytic
Terbutaline (Brethine)
Also called Brethine, Bricanyl
Indications
- Prevention and reversal of bronchospasm from asthma, bronchitis or emphysema, by SC injection at age 12 and up (label)
- Severe asthma when inhaled drug cannot get in; IV infusion is off-label in the US and not routine in adults
- Uterine tachysystole with a worrying tracing after oxytocin is stopped, 0.25 mg SC (off-label)
- Tocolysis for up to 48 h to allow steroids (off-label; boxed warning beyond 48–72 h)
- Vasopressor extravasation when phentolamine is unavailable, infiltrated locally (off-label)
Dosing
Terbutaline drip card and mL/h converterStart: Severe asthma when inhaled drug cannot get in: 0.25 mg SC in the lateral deltoid, once more in 15–30 minutes, max 0.5 mg in 4 hours (label; EPR-3 allows every 20 minutes × 3). IV infusion (off-label in the US, not routine in adults): 1.5–5 mcg/min (UK label)
Mechanism of action
Terbutaline is a selective β₂-adrenergic agonist. The β₂ receptor couples to Gs, so adenylyl cyclase raises cAMP and protein kinase A phosphorylates targets that lower intracellular calcium, inhibit myosin light-chain kinase and open large-conductance K⁺ channels, relaxing bronchial, uterine and vascular smooth muscle. The same pathway stimulates Na⁺/K⁺-ATPase, which drives potassium into cells, and increases glycogenolysis. Selectivity is relative: at higher doses β₁ stimulation adds tachycardia.
Pharmacology
- Respiratory: bronchodilation within 15 min of a SC dose; less mast cell mediator release
- Cardiovascular: tachycardia from vasodilation and, at higher doses, β₁ stimulation; palpitations, arrhythmias, myocardial ischemia
- Metabolic: hypokalemia, hyperglycemia and lactic acidosis, which can mimic worsening asthma
- Uterine: relaxes the myometrium, the basis of short-term tocolysis
- Neuromuscular: tremor
- Fetal and neonatal: crosses the placenta, causing fetal tachycardia and newborn hypoglycemia
Pharmacokinetics
| Onset | SC: measurable bronchodilation within 5 min, clinically significant within 15 min (label) |
|---|---|
| Peak | 30–60 min after SC (label) |
| Duration | 1.5–4 h (label) |
| Half-life | About 5.7 h in healthy adults, 2.9–14 h in patients (US label); terminal 16–20 h (UK label) |
| Distribution | Vd about 114 L, roughly 1.6 L/kg (UK label); crosses the placenta |
| Metabolism | Sulfate conjugation, the main metabolite; the resorcinol ring is not a COMT substrate |
| Elimination | About 90% in urine within 96 h, roughly 60% unchanged (label) |
| Routes | SC injection and oral tablets in the US; IV and inhaled forms elsewhere |
Chemistry
- (±)-5-[2-(tert-butylamino)-1-hydroxyethyl]benzene-1,3-diol as the sulfate salt (2:1), MW 548.65 (label)
- The resorcinol ring escapes COMT and the tert-butyl group on the amine favors β₂ receptors, giving a longer, more selective action than isoproterenol
- Racemic; the R-enantiomer carries the β₂ activity
- Hydrophilic amine, pKa about 8.7 (PubChem), mostly ionized at pH 7.4
- 1 mg/mL in sodium chloride, pH 3–5; protect from light and discard if discolored (label)
Adverse effects and cautions
In the one-pagers
- Asthma and COPD ExacerbationsBronchodilators, steroids, magnesium and parenteral β-agonists: Epinephrine or terbutaline: 0.3–0.5 mg or 0.25 mg SC every 20 min ×3: When inhaled drug cannot get in
- Vasopressors and InotropesLines, extravasation and titration: Extravasation: stop the infusion, aspirate, remove the catheter, mark and elevate. For catecholamines or phenylephrine, infiltrate phentolamine 5–10 mg in 10–15 mL saline subcutaneously as soon as possible (label: within 12 h). Alternatives: terbutaline or topical nitroglycerin.
- CEN: Genitourinary, Gynecologic and Early-Pregnancy EmergenciesExam pearls: Priapism over 4 h is a urology procedure, not oral terbutaline and a wait.
- Obstetric EmergenciesDrugs in pregnancy in the ICU: Betamethasone (12 mg IM, two doses 24 h apart) for likely delivery before 34 weeks raises glucose and WBC for days, mimicking infection. Magnesium before 32 weeks cut moderate or severe cerebral palsy from 3.5% to 1.9%, a secondary outcome (BEAM, Rouse 2008). Rho(D) immune globulin 300 mcg covers 30 mL of fetal whole blood. Terbutaline carries an FDA boxed warning ag
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Sources
- FDA label: Terbutaline Sulfate Injection (Fresenius Kabi, 2022)
- UK SmPC: Bricanyl Injection (2022)
- FDA safety communication on terbutaline for preterm labor (2011)
- PubChem: terbutaline
For education and exam preparation only. Follow your institution’s protocols. Typical adult values for study. Follow the order, the label and your institution's protocols.