Drug given to reduce the uterine contraction during preterm labour with LEAST side effect is:

Administration of progesterone inhibits the secretion of pro-inflammatory cytokines and delays cervical ripening. Thus, progesterone and its derivatives have long been advocated to diminish the onset of preterm labor in women at increased risk due to previous preterm delivery. Specific tocolytic agents include beta adrenergic receptor agonists, MgSO4, Ca2+ channel blockers, COX inhibitors, oxytocin-receptor antagonists, and nitric oxide donors. Among the given options progesterone is the drug with the least side effect. Ref: Schimmer B.P., Parker K.L. (2011). Chapter 66. Contraception and Pharmacotherapy of Obstetrical and Gynecological Disorders. In L.L. Brunton, B.A. Chabner, B.C. Knollmann (Eds), Goodman & Gilman's The Pharmacological Basis of Therapeutics, 12e.