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pack should be started with the first day of your menses that is the day when you have first bleed, you can start in first 5days also.
The next cycle will start itself within 3-5days after you have completed 21 pills.
Regularity with one pill a day is the most important factor to improve efficacy.
If you have any further query please let me know I would try to answer it for you.
Wish you a great health!