Emergency contraceptive pills contain estrogen, a hormone that could potentially decrease your milk supply or, rarely, make your milk disappear entirely. Still, one-time use isn’t likely to have a huge effect on your supply or your baby.
There haven't been any long-term studies on the health effects of taking this drug repeatedly, so only use it for its intended purpose: as emergency contraception. Use a reliable form of contraception to avoid such an emergency. Barrier contraceptives (like condoms) are considered safest while breastfeeding. If opting for a hormonal form of birth control, it is best to go with a progestin-only pill (mini-pill) or patch.