Mechanism of action

Combined oral contraceptives (COCs), most commonly referred to as “the pill”, 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 to decrease endometrial hyperplasia which cuts the chance of implantation. 

The Faculty of Sexual and Reproductive Health (FSRH) says that nine per cent of individuals who use a COC “typically” will experience an unintended pregnancy within the first year of use. This falls to 0.3 per cent when taken “perfectly” (i.e. consistent and correct use). 

These rates are the same as for other combined contraception methods such as the vaginal ring and transdermal patch, but considerably lower than Long Acting Reversible Contraception (LARC) methods, such as implants and intrauterine devices and systems, as there is little scope for user error. COCs compare favourably to methods such as condoms (18-21 and two to five per cent for typical and perfect use respectively), the withdrawal method (22 and four per cent), and fertility awareness (24 and one to nine per cent).