Provided that women return every 12 weeks(8 weeks for Noristerat®) for their injection,
there is a very low failure rate in studies- around 2 per 1,000 women per year.
Provided that the injections are given on a regular basis(every 12 weeks for Depo-Provera®,
every 13 weeks for Sayana Press®, every 8 weeks for Noristerat®), there is a very low failure rate.