Other tocolytics

Nitric oxide donors, progesterone, and magnesium have all been tested for their possible tocolytic properties.

Nitroglycerine is the only nitric oxide donor to be tested in clin­ical trials and acts by causing relaxation of the uterine muscle.

Progesterone is normally used as a prophylactic agent to prevent the onset of PTB; however, it has been used increasingly for acute tocolytic properties. In a randomized, double-blind controlled trial, there was no difference in neonatal morbidity when 200 mg vaginal progesterone was compared with placebo for tocolysis (RR 1.2; 95% CI 0.82-1.8) (84).

Magnesium sulphate is the most commonly used tocolytic in the United States. Extracellularly, it suppresses calcium influx into myometrial cells. Intracellularly, it competes with calcium inhibiting myosin light chain activity. As with other tocolytics contradictory data exist with regards to effectiveness. A Cochrane review failed to prove any delay in PTB when compared with no therapy or pla­cebo (85), but a network meta-analysis found it to be the second most effective drug in delaying PTB for 48 hours (RR 2.76; 95% CI 1.58-4.94) (79). It has a poor side effect profile causing intense flushing and also has to be administered intravenously. Magnesium sulphate is frequently given in PTL for neonatal neuroprotection and there are no known contraindications to giving other tocolytics concomitantly.