However, that is not the primary mechanism of any of these forms — not even the hotly contested Plan B. The newest research indicates that all of these methods primarily interfere with fertilization, whether by blocking sperm, immobilizing them, or inhibiting ovulation. This doesn’t mean that they never cause a fertilized egg to fail to implant. It’s remotely possible that they do — which is why some labels say so, just as tampon labels indicate that tampons may cause Toxic Shock Syndrome.