Final answer:
Magnesium sulfate is the medicine used to slow down uterine contractions and stop preterm labor.
Step-by-step explanation:
The medicine used in labor and delivery patients to slow down uterine contractions and stop preterm labor is Magnesium sulfate. Magnesium sulfate is a tocolytic agent that works by relaxing the smooth muscles of the uterus, therefore slowing down contractions. It is commonly used in cases of preterm labor to delay delivery and give the baby more time to develop.