Norethindrone molecule. Computer graphic of a molecule of the contraceptive hormone norethindrone (chemical formula: C18.H26.O2). The atoms are represented as spheres, and are colour coded: carbon is green, hydrogen is white and oxygen is blue. Norethindrone has a similar mode of action to the natural female hormone progesterone, and it is commonly used in contraceptive pills. It is taken either by itself, or combined with a synthetic oestrogen. These oral contraceptives interfere with the body's natural hormone balance to prevent ovulation, and hence the risk of pregnancy.