In the large intestine, a proportion of the conjugates are split by hydrolytic enzymes released
from colonic bacteria (Clostridia, Bacteroides species, lactose-fermenting coliforms and
some Staphylococci). Conjugates are split, releasing the free drug or metabolite, which can
then be reabsorbed. These are eventually excreted in urine.
The degree of reabsorption of EE via the enterohepatic circulation may vary between
individuals. There have been theoretical concerns about the effect that this reabsorption of
EE may have in terms of contraceptive efficacy but to date it is unproven.
There is no enterohepatic circulation of progestogens in their active forms and thus
contraceptive efficacy is unaffected by changes in gut flora.