If some one have un protected sex on the day when the girl was expecting periods, then can she take emergency contraceptive pregnancy tablet?/ will it effect her periods? What will be the side effect?

Morning-After Pill (Emergency Contraception) at a Glance
Birth control you can use to prevent pregnancy up to five days (120 hours) after unprotected sex
Two kinds of emergency contraception — morning-after pill and IUD insertion
Safe and effective
Available at health centers and drugstores
Costs vary from $10 to $70 for the morning-after pill and up to $500 for IUD insertion

I will add that most morning after pills are less effective the longer you wait, with most effect within the first 72 hours.

They have many side effects, and yes they will effect periods