Plan B is not bad for you. It is safe to take every time you have unprotected sex, the birth control method failed, or you forgot to take birth control. It’s better to use long term birth control (such as the pill, patch, ring, IUD, implant) if you find yourself using Plan B and its generics (Levonorgestrel) more than twice.
There are no dangers (as in death, and the side effects have been covered by the other answers) to taking the morning-after pill too often. EXCEPT that it might not work and you end up getting pregnant. It’s efficacy depends on where a woman is in her cycle.
So it is NOT recommended to take too often, simply because we hope you would use a more efficacious form of birth control if you really want to avoid pregnancy.
Side effects of the morning after pill include: nausea, vomiting, menstrual irregularities, dizziness, diarrhea, and/or breast tenderness.
If you have had an allergic reaction to levonorgestrel or “the morning after pill” before, do NOT take it.