What is it used for? Contraception How does it work? Mercilon tablets are a type of hormonal contraception commonly known as 'the pill' or combined oral contraceptive pill. Mercilon tablets contain two active ingredients, ethinylestradiol and desogestrel. These are synthetic versions of the naturally occurring female sex hormones, oestrogen and progesterone. Ethinylestradiol (previously spelt ethinyloestradiol in the UK) is a synthetic version of oestrogen and desogestrel is a 'third generation' synthetic form of progesterone. Combined oral contraceptives like Mercilon work by over-riding the normal menstrual cycle. In a woman's normal menstrual cycle, levels of the sex hormones change throughout each month. The hormones cause an egg to be released from the ovaries (ovulation) and prepare the lining of the womb for a possible pregnancy. At the end of each cycle, if the egg has not been fertilised the levels of the hormones fall, causing the womb lining to be shed as a monthly period. The daily dose of hormones taken in the pill work mainly by tricking your body into thinking that ovulation has already happened. This prevents an egg from ripening and being released from the ovaries each month. The hormones also increase the thickness of the natural mucus at the neck of the womb, which makes it more difficult for sperm to cross from the vagina into the womb and reach an egg. They also change the quality of the womb lining (endometrium), making it less likely that a fertilised egg can implant there. Mercilon is a monophasic pill. This means that each tablet has the same dose of hormones in it. One tablet is taken every day for 21 days and you then have a seven day break from pill-taking. During your seven day break, the levels of the hormones in your blood drop, which results in a withdrawal bleed that is similar to your normal period. You start the next pack after the seven pill-free days are up, even if you are still bleeding. The tablets come in a calender pack marked with days of the week to help you remember to take a pill every day for three weeks, followed by a week off. You will still be protected against pregnancy in your pill-free week, provided you took all the pills correctly, you start the next packet on time and nothing else happened that could make the pill less effective (eg sickness, diarrhoea, or taking certain other medicines - see below). The contraceptive pill results in lighter, less painful and more regular menstrual bleeding. This means it is sometimes also prescribed for women who have problems with particularly heavy, painful or irregular periods.