Mechanism of action

Combined oral contraceptives (COCs) contain oestrogen and progesterone. Both act on the hypothalamo-pituitary axis to reduce production of luteinising hormone and follicle-stimulating hormone in order to suppress ovulation.

The two hormones also combine to thicken cervical mucus, making it more difficult for fertilisation to occur, and decrease endometrial hyperplasia, which reduces implantation.

The FRSH states that 9 per cent of individuals who use a COC “typically” will experience an unintended pregnancy within the first year. This falls to 0.3 per cent when taken “perfectly” (i.e. consistent and correct use). This is the same as for other combined contraception methods such as the vaginal ring and transdermal patch. The figures for LARC methods such as implants and intrauterine devices are considerably lower, as there is little scope for user error.

Combined oral contraceptives compare favourably to methods such as condoms (18-21 per cent and 2-5 per cent experiencing unintended pregnancy with typical and perfect use respectively), the withdrawal method (22 per cent and 4 per cent) and fertility awareness (24 per cent and 1-9 per cent).