How effective is the shot at preventing pregnancy?

As long as you get your shots on time, this method is very effective. With perfect use the shot is more than 99% effective; with typical use, the it’s closer to 94%.

In other words:

Of those women who use the shot exactly as directed, fewer than 1 in 100 will experience an accidental pregnancy during the first year of using this method.

Of those women who do not use the shot exactly as directed, fewer than 6 in 100 will experience an accidental pregnancy during the first year of using this method.