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About COCs
COCs contain an oestrogen and a progestogen, and work primarily by inhibiting ovulation through acting on the hypothalamo-pituitary-ovarian axis to reduce secretion of luteinising hormone and follicle stimulating hormone; they also have effects on cervical mucus and the endometrium. 5 COCs are available in either 21-day packs with a 7-day break between each pack, or as 'Every Day' packs of 21 tablets with active ingredients, followed by seven pills with no active ingredient, taken without a break between packs. During the 7 pill-free or inactive-pill days, the endometrium is shed, resulting in a withdrawal bleed in most women. 5 In addition, unscheduled bleeding or spotting can occur with COC use, and these symptoms, especially if they are irregular, prolonged or frequent, may lead to discontinuation of the pill. 6 Such bleeding is more common with a 20µg ethinyl estradiol (EE) COC than with COCs containing more than 20µg EE. 7
Some COCs contain a fixed amount of an oestrogen and a progestogen in each active tablet and are termed 'monophasic' (e.g. Microgynon). Others have varying amounts of the two hormones according to the stage of the cycle and are termed 'phasic' (e.g. biphasic such as BiNovum, or triphasic such as Triadene). 8 Previously in the UK, the oestrogen component of available COCs has been EE with various progestogens such as norethisterone or levonorgestrel (so-called second-generation COCs), or desogestrel or gestodene (third-generation COCs).
COCs are generally used by healthy women for preventive purposes; they therefore need to have a well-defined contraceptive efficacy and a very low risk of harm to offer a favourable risk/benefit balance in such individuals. 9
Contraceptive efficacy can be measured using the Pearl Index (the number of unintended pregnancies per 100 woman-years of exposure), which can be expressed in two ways: 'user and method failure' (also called the unadjusted Pearl Index). This represents the failure rate with 'typical' use and includes all pregnancies and all cycles of contraceptive use, except those in which additional methods have been used. The rate is around 12% for condoms; 3% for IUDs and 3% for pills; 10
'method failure' (also called 'true pill failure' or the adjusted Pearl Index). This represents the failure rate with 'perfect' (i.e. correct and consistent) use and excludes...