Oral contraceptive "potencies" and deep venous thromboembolism [letter]

The view is expressed contrary to 2 similar papers purporting to demonstrate an association between the potency of oral contraceptives and the incidence of deep vein thrombosis. The present writer states that it is not feasible to derive a potency scale for oral contraceptives because pills are formulated from combinations of estrogens and progestogens each of which have complex qualitative and pharmacologic effects and interactions. The objections raised against the papers include citing a set of estrogen potencies derived from treating human endometrium with mestranol and ethinyl estradiol misquoting the source of the original work confusing dose with potency using data on potency of progestins that could not later be reproduced and using unreliable secondary sources for other progestational potencies.