What is the onset of action of nifedipine (Procardia) 20mg PO for tocolysis?

Onset of Action of Nifedipine (Procardia) 20mg PO for Tocolysis

Nifedipine begins working within 10-30 minutes when administered orally for tocolysis, with peak plasma concentrations occurring at approximately 2.5-5 hours after ingestion. 1

Pharmacokinetics of Nifedipine in Tocolysis

• Nifedipine is completely absorbed after oral administration, with bioavailability of 84-89% compared to immediate release formulations 1
• When administered orally for tocolysis, nifedipine reaches peak plasma concentrations between 2.5-5 hours, with a second smaller peak at 6-12 hours post-dose 1
• The elimination half-life of nifedipine is approximately 7 hours when administered as extended-release tablets, compared to 2 hours for immediate-release formulations 1
• Maternal half-life of nifedipine during tocolysis averages 81 minutes (range 49-137 minutes) 2

Onset of Action for Tocolytic Effect

• Initial effects on uterine activity begin within 10-30 minutes of oral administration 3
• Sublingual administration may provide faster onset of action compared to oral administration for tocolysis 4
• Studies show that sublingual nifedipine results in faster tocolysis (160 minutes) compared to oral administration (340 minutes) 4
• Peak plasma concentrations during sublingual therapy for tocolysis range from 23.4 to 197.9 ng/ml, showing substantial interpatient variability 2

Dosing Recommendations for Tocolysis

• Standard loading dose is 20 mg orally, which may be repeated every 30 minutes up to a maximum dose of 60 mg 4
• Maintenance dosing typically involves 10-30 mg orally every 6-8 hours 5
• For acute tocolysis, some protocols use 10 mg sublingual nifedipine for faster onset of action 5
• Mean measurable trough value in patients receiving 20 mg of nifedipine orally every 6 hours is 7.2 ± 5.5 ng/ml 2

Clinical Considerations and Precautions

• Blood pressure monitoring is essential during nifedipine administration for tocolysis due to its vasodilatory effects 3
• Nifedipine should not be administered sublingually due to risk of precipitous blood pressure decline 3
• Caution is advised when combining nifedipine with magnesium sulfate, as this combination may cause precipitous blood pressure drops 3
• Nifedipine crosses the placenta, with measurable levels found in some neonates at delivery 2

Efficacy Considerations

• Success rates for tocolysis with nifedipine (defined as delaying delivery by more than 48 hours) have been reported at 84% in some observational studies 6
• However, a randomized controlled trial showed no significant difference between nifedipine and placebo in preventing preterm birth before 37 weeks (52% vs 48%) 7
• The same trial found no significant difference in delaying delivery by at least 48 hours (78% with nifedipine vs 71% with placebo) 7

Common Side Effects

• Flushing, headache, and rarely hypotension (especially in hypovolemic patients) 5
• Reflex tachycardia may occur due to peripheral vasodilation 3
• Maternal and fetal side effects are generally minimal at tocolytic doses 6