Retosiban is a small molecule oxytocin receptor antagonist that is under evaluation for treatment of premature labor. Retosiban was found to be safe in healthy non-pregnant volunteers in phase I studies. Intravenous retosiban also had good safety and tolerability in phase II studies and was suggested to prolong pregnancies in women with preterm labor. Phase III studies have been conducted to demonstrate the efficacy of retosiban to prolong pregnancy and improve neonatal outcomes, and compare effects with a similar drug atosiban, but these studies were terminated in 2018 (not due to adverse events).