Nearly all forms of birth control are effective for women regardless of weight.
The one important exception is emergency contraception. Research shows that levonorgestrel EC (like Plan B and Next Choice) and ella are less effective for obese women. How much less effective? For these women, taking an EC pill is the same as doing nothing. However, there is an EC that works for women of any weight: the copper IUD. It’s the most effective form of EC—including for women who aren’t obese.