The norethindrone minipill is over 99% effective as birth control when taken correctly. If you sometims forget a pill or take it more than 3 hours late it's 91% effective. There aren't specific stats for FTM people, but doctors still recommend that they take emergency contraception if there's been a birth control mistake.

There's another type of pill, norethindrone acetate 5 mg, that's sometimes prescribed to stop period but hasn't been studied as birth control. But if you asked your doctor for a birth control prescription they shouldn't have given you that. Just specifying because recently a couple of people who posted there where using hormonal treatment for their periods as birth control.