First thing first, what is an Emergency Pill? It is a form of contraceptive taken to prevent pregnancy or conception after an unprotected sex.
The very common ones among few are: Postinor-1 or 2, Postpill or PlanB One Stop; its chemical constituent is Levonogestrel 0.75 or 1.5mg while combined pill contains ethinyl estradiol 0.03 with levonogestrel 0.15 which is commonly referred to as Combination-3 or Everyday Pill.
How does an Emergency Pill prevent pregnancy?

Conception is a process where an egg is fertilized by a sperm. An Emergency Pill acts by delaying the release of egg from ovary into the fallopian tube where a sperm is potentially waiting to meet with the released egg and when an egg is not released, conception cannot take place.
It also acts by preventing implantation of fertilized egg. How does it do this? It makes the uterine wall (endometrium) unreceptive to the fertilized egg. Reason Emergency Pill has up to 95% efficacy.
Why use an Emergency Pill?

You had an unprotected vaginal sex without condom, ring or copper IUD.
Your partner had a condom-cut in the process.
You had an unprotected vaginal sex unwillingly.
What Emergency Pill is best for me?

Here are factors to consider:
-Are you currently on any of the hormonal birth control pills?
If yes, discontinue before switching to an Emergency pill because interaction could exist between two different pills taken almost at same time.
-Are you a nursing mother?
Taking an Emergency pill in breastfeeding is likely to upset hormonal imbalance in mothers. Consult your doctor to determine risk/benefit ratio.
-Are you obese with high BMI (Body Mass Index)?
Studies have shown Emergency pill to be less effective in Obese, rather other birth control method is suggested.
-Are you treating or managing any medical symptom or condition?
See your doctor for further consultation.
-Cost effectiveness and easy access needs to be factored in as well.

When do I take Emergency Pill?

It is to be taken within 72 hours after having an unprotected sex; however it is best taken as soon as possible after an unprotected sex.
How often can I take Emergency Pill?

All forms of hormonal or non-hormonal contraceptives are quite safe and can be taken as many times as possible with little or temporary side effects like nausea or inconsistent menstrual flow or cycle.
In conclusion, it is noteworthy to mention here that most contraceptives does not replace an abortion pill, it only prevent pregnancy or conception from taking place.
Birth control pills; Ring, Shot, Patch, and IUD could be more affordable and convenient for an active sex life to taking Emergency Pill each and every time.