Hi
There are two types of Midol: one contains acetominofen (Tylenol) plus caffeine, and one contains naproxyn plus caffeine.
Both are safe during breastfeeding. You should be aware that caffeine sometimes causes cystic breasts. However, it is not dangerous or harmful to the baby with breastfeeding.
http://www.mayoclinic.com/health/drug-information/DR602345
Thanks
MD
Experienced Physician Specialising in Medicine,Gynae,ER