Oral contraceptive pills and their packaging; each pill is marked with a date to prevent an omission of the daily dose. Contraceptive pills contain one or more of synthetic female sex hormones. Many types are a combination of an oestrogen, which blocks the process of ovulation (release of mature ovum - egg), and a progestogen, which acts on the pituitary gland to affect the normal menstrual cycle. Progestogens also increase viscosity of mucous in the cervix (neck of womb), providing a barrier to sperm should ovulation occur. Side- effects are numerous, including weight increase, nausea, depression, and a risk of thrombosis.

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