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Contraceptive pills. Assorted oral contraceptive pills used for birth control. Dates are present on the packs of 20 to ensure that the correct dose is taken & to prevent omission of the daily dose. The contraceptive pill contains one or more synthetic female sex hormones. Most contraceptive pills consist of both a progesterone & an oestrogen. Progesterone acts on the pituitary gland affecting the menstrual cycle, & oestrogen blocks the monthly release of a mature egg (ovulation). The unwanted pregnancy rate is less than 1 per 100 woman years. Side effects from using the pill include headache, weight gain, nausea & higher risks of cervical cancer & thrombosis.