Progesterone has a quieting effect on smooth muscle contractility.

Largely for this reason, gastrointestinal function usually slows to some degree during the second half of the menstrual cycle.

While most women do not notice the change, a few will notice bowel sluggishness, constipation, increased gas production and abdominal dissension. While this is not dangerous, it can be annoying. When combined with the natural tendency in many deployed settings to intentionally dehydrate (avoiding the problem of urination), constipation can become a quite significant problem.

BCPs can block this change in gastrointestinal function by virtue of the inhibition of ovulation and the hormone changes that go along with ovulation. Increasing dietary fiber and fluid intake can also be helpful. In extreme cases in operational settings, bulk laxatives or bowel stimulants may prove necessary.