Mercilon is one of a group of medicines called combined oral contraceptives ("the Pill"). You take it to prevent pregnancy. When used as instructed in the patient leaflet you are very unlikely to become pregnant.

You must take one Mercilon pill once a day for 21 days in a row.  Mercilon contains a small amount of the female sex hormones, estrogen and progestogen.  These hormones prevent an egg being released from your ovaries.  If your ovaries do not release an egg you cannot become pregnant. 

The 21 days of taking a pill once a day are followed by 7 days when you don't take a pill.  During these pill-free days you will have your monthly period.  However, this period may be lighter and shorter than usual.  This is called a withdrawal bleed.