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, that means how real people use it in real life, it’s closer to 96%.

In other words:

For people who use the shot exactly as directed for a whole year, fewer than 1 in 100 will get pregnant when they don’t want to during the first year of using the shot.

For people who use the shot as people typically use it, fewer than 4 in 100 will get pregnant when they don’t want to during the first year of using the shot.”