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pill is good method for cotraception at your age,as Gynecologist has already examined you and suggested you can start the beinge coloured tab on first day of your menstruation
as it take few days to give protection and you will have protection from begning and you have to take those placebo or Iron tablets after 21 days and restart new pack after that.
Iron supplement helps to prevent Anemia
and you do not forget to take your pill.
If you start late then you may have to use other contraceptive methods for 7 days.
In case of any abnormality please check up with your Gynecologist.