Can postinor fail if it took them twice before 12 hrs?

Postinor is an emergency contraceptive containing hormones that prevent ovulation and interfere with sperm/egg transport. It is 85% effective. It should be used only rarely as it disrupts your normal hormone balance in unpredictable ways. If you are using it this often, you should consider a different type of contraceptive (oral contraceptives, depo provera shot, hormonal Implant). Talk to your DR