If you have already conceived a baby, but the baby has not implanted yet, Plan B will cause an abortion. The manufacturer claims that there is no evidence that the drug will harm an implanted pregnancy. It’s generally gravely immoral to use it, however, many Catholic theologian argue that it doesn’t go against Church teaching to use it in the case of a rape, provided that it is known that the woman hasn’t ovulated and there is no risk of an abortion. I don’t think there is an official teaching regarding that situation, but the teachings of the Church are clear that abortion is always gravely immoral and the use of contraception in a marriage is also gravely immoral.
According to the manufacturer’s website, it is simply a larger dosage of a commonly used BC synthetic hormone, so it isn’t likely to cause more long-term damage then regular hormonal birth control, unless you are literally taking it every other day or something like that.