What is the recommended dose of isoxuprine (isoxuprine hydrochloride) for a pregnant woman between 24 and 34 weeks of gestation with a singleton pregnancy and no contraindications to control preterm labor?

Isoxsuprine Hydrochloride Dosing for Preterm Labor

Isoxsuprine is NOT recommended as a first-line tocolytic agent for preterm labor, as it has been largely replaced by safer and more effective alternatives such as nifedipine and atosiban, which are supported by current guidelines. 1, 2, 3

FDA-Approved Dosing (When Used Off-Label for Preterm Labor)

The FDA-labeled oral dose of isoxsuprine is 10 to 20 mg, three or four times daily. 4 However, this dosing was established for peripheral vascular disease, not specifically for preterm labor management.

Clinical Dosing Regimens Used in Practice

When isoxsuprine has been used for preterm labor (though not guideline-recommended), the following regimen has been reported:

• Intravenous infusion: 40 mg isoxsuprine hydrochloride infused until uterine quiescence is achieved 5
• Intramuscular maintenance: 10 mg every 4 hours for the first 24 hours 5
• Oral maintenance: 40 mg sustained-release capsules twice daily until delivery or 37 completed weeks 5

Alternative oral dosing reported in older studies includes 10-20 mg orally every 8 hours. 6

Why Isoxsuprine Should NOT Be Your First Choice

Current guidelines from the European Society of Cardiology and American College of Obstetricians and Gynecologists do NOT recommend isoxsuprine as a first-line tocolytic. 1, 2 The preferred agents are:

• Nifedipine (calcium channel blocker): 10-20 mg oral loading dose, with maintenance dosing up to 120-160 mg daily 2, 7
• Atosiban (oxytocin receptor antagonist): For women between 24-34 weeks gestation 3
• Intravenous labetalol or hydralazine: When severe hypertension complicates preterm labor 1

Significant Safety Concerns with Isoxsuprine

Isoxsuprine carries substantial maternal cardiovascular risks that limit its clinical utility:

• Maternal tachycardia is common (8% incidence) and may require dose adjustment 5
• Severe hypotension can necessitate discontinuation of therapy 8
• Pulmonary edema has been reported, particularly when combined with corticosteroids and excessive fluid administration 9
• Higher failure rates (22.22%) compared to alternative tocolytics like ritodrine (6.5% failure rate) 6

Comparative Efficacy Data

When isoxsuprine has been compared to nifedipine:

• Nifedipine achieved 81.25% successful tocolysis versus 70% with isoxsuprine 8
• Mean pregnancy prolongation was 25 days with nifedipine versus 19 days with isoxsuprine 8
• Maternal side effects were similar between both agents, but nifedipine is now guideline-supported 8, 2

Critical Clinical Pitfalls to Avoid

• Do not use isoxsuprine when guideline-recommended alternatives (nifedipine, atosiban) are available 1, 2, 3
• Monitor fluid balance meticulously if isoxsuprine must be used, as pulmonary edema risk increases with positive fluid balance exceeding 5 liters 9
• Avoid combining with corticosteroids without careful fluid restriction, as this combination increases pulmonary edema risk 9
• Discontinue immediately if signs of pulmonary edema (dyspnea, chest pain, wet rales) develop 9

The Appropriate Modern Approach

For a pregnant woman between 24-34 weeks with preterm labor and no contraindications, the evidence-based approach is:

1. First-line tocolysis: Oral nifedipine 10-20 mg loading dose, with maintenance up to 120 mg daily 2, 7
2. Concurrent corticosteroids: Betamethasone or dexamethasone for fetal lung maturation if delivery anticipated before 35 weeks 1
3. Magnesium sulfate: For fetal neuroprotection before 32 weeks 3
4. Goal of tocolysis: Delay delivery 48-72 hours to allow corticosteroid administration and maternal transfer, not to prevent preterm birth entirely 3

Isoxsuprine should only be considered in resource-limited settings where guideline-recommended tocolytics are unavailable. 8