Post 125 – Ask Prof: oral contraceptives (the pill)

Student asked me: how does the pill work? (a male student, no less) So I gave him the basics on what is in the pill and the feedback to the brain to eliminate the LH surge that causes the egg to be released at ovulation. So let’s go through the basics and the rest of the story:

The Basics

Oral contraceptives (abbreviated “oc” in medical jargon, and I’ll abbreviate it “OC” in this blog) are pills that are taken daily to prevent pregnancy. The most common ones contain some combination of the sex hormones estrogen and progesterone. (Do you know that men ALSO have these hormones?)  There are some progesterone-only pills, so the estrogen is not required.

We’ll talk about the ones with estrogen and progesterone (called combination pills) because I’m more familiar with these (don’t ask me why). Pills containing some combination of these hormones are taken for 21 days, and then the woman either takes a placebo pill (included in the package) or no pills for 7 days, before the cycle begins again.


Progesterone works on the Hypothalamus by a negative feedback loop

Normally a releasing hormone (specifically gonadotropin-releasing hormone-GnRH) is leaves the hypothalamus telling the anterior pituitary gland to release follicle stimulating hormone (FSH) and luteinizing hormone (LH). The constant supply of progesterone from the pill prevents this GnRH from increasing LH, thus you don’t have the LH surge to cause the egg to be released.

Having progesterone not only reduces LH but it also reduces FSH, which reduces follicle growth (a chamber in the ovary where the egg grows).

So, to summarize, because I know how you like bullet-points –

No LH surge because:

  • Progesterone exists in the bloodstream (comes from the pill)
  • No increase in estrogen
  • So by negative feedback to the hypothalamus – reduces the amount of LH and FSH coming from the anterior pituitary.

Estrogen is included in some OC to stabilize the endometrium of the uterus (reduce bleeding outside the menstruation window).


Other OC effects

Progesterone thickens cervical mucus, which keeps sperm from entering the uterus, and thins the lining of the uterus, which may interfere the blastocyst from implanting to the endometrium.

Estrogen, included in some pills, not only helps to prevent bleeding from the uterus outside menstruation, but it also inhibits follicular development and helps prevent ovulation. “Estrogen negative feedback on the anterior pituitary greatly decreases the secretion of FSH, which inhibits follicular development and helps prevent ovulation” (Wikipedia)


You CAN get pregnant while on OC

Yes. They state OCs are 99% effective. So how can you get pregnant?

Most common reason is that someone misses a pill. You’re supposed to take it the same time every day to keep concentrations as close to stable as possible day-to-day (plus hopefully it helps you remember to take it). Modern OCs have the synthetic hormones in such low doses that the amounts may not in effective amounts after 24 hours.

Fox News in a 2008 article states 5 reasons a pregnancy can occur. We already talked about missing a dose and not taking the pill at the same time every day. The remaining three are:

  • Alcohol use. OCs are metabolized by the liver to the active hormone forms. Alcohol, also metabolized by the liver, may reduce how effective the liver is in “activating” the OC.
  • Antibiotics and seizure medications
  • Taking generic forms of the pill – The FDA allows for a 15% variation in generic medications. Because the current pills are so low-dose, the concentrations may be too low to be effective.

Hope that helps!




Have you heard?

An arrest warrant has been issued for a man in Stockton who has TB – not for a crime, but because he has TB!

There’s so much I’d like to write up this week, but I don’t have the time. Keep up with current medical news (that I wish I can write on this blog) on Twitter @prof_chang


One comment

  1. News today highlights an medical journal article showing a higher risk for breast cancer among those who take high-dose estrogen oral contraceptives. As noted in the original post, the trend is for lower dose, but it brings a higher risk for pregnancy, especially if someone misses a dose. Some doctors say this higher risk for breast cancer in the high-dose estrogen oral contraceptives is less than 1%, so the benefits of not getting pregnant outweigh this small higher risk of breast cancer.

    Original source article can be found here ( National Cancer Institute also has a page on this (

    Newsclip from CBS News (

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