Which medication among the following is a calcium channel blocker used as a tocolytic to suppress preterm labor?

• A. Indomethacin
• B. Nifedipine/Procardia
• C. Magnesium sulfate
• D. Terbutaline

Answer: (B)

To relax the uterus in preterm labor, you want a medication that directly reduces uterine smooth muscle contraction by limiting calcium entry. Calcium is essential for muscle contraction; blocking the calcium channels in smooth muscle prevents calcium from entering the cells, lowering intracellular calcium and causing the myometrium to relax. A calcium channel blocker fits this action exactly, so it’s used as a tocolytic to suppress contractions. Among the options, this drug stands out because it works by inhibiting calcium influx, whereas the others work through different mechanisms (indomethacin inhibits prostaglandin synthesis; magnesium sulfate is mainly for seizure prophylaxis with some tocolytic effect via neuromuscular blockade; terbutaline is a beta-adrenergic agonist that relaxes the uterus through another pathway). Use of nifedipine requires monitoring for low blood pressure, since it can cause hypotension, and be cautious with other antihypertensives.