Why does diarrhea affect birth control?

Diarrhea can affect how well birth control pills work. This is because diarrhea can speed up the passage of food and fluids through your body, potentially preventing your body from fully absorbing the hormones in the pill. As a result, the effectiveness of your birth control can be reduced if you experience diarrhea or vomiting soon after taking your pill.

How does diarrhea interact with birth control pills?

Birth control pills contain estrogen and progestin hormones that prevent ovulation and thicken cervical mucus to block sperm from reaching an egg. For the pill to work properly, you need consistent hormone levels in your bloodstream. When you have diarrhea, the hormones from your pill can pass through your body faster than normal before being fully absorbed. This can lower the hormone levels in your body, making your birth control less effective.

Vomiting can also prevent your body from absorbing the hormones in your pill. So if you experience vomiting along with diarrhea shortly after taking your pill, the effectiveness is further reduced.

How long do the effects last?

The effects of diarrhea and vomiting on birth control can last as long as you continue having diarrhea or vomiting episodes. Each time you experience diarrhea or throwing up within 2 hours of taking your pill, you are at risk of lowered hormone absorption and birth control effectiveness.

Once the diarrhea stops, it takes about 2 days for the hormone levels in your body to build back up. So the risk remains higher for about 48 hours after your last episode of diarrhea or vomiting.

Does the timing matter?

Yes, the timing of the diarrhea in relation to when you take your pill matters. Here’s why:

  • Diarrhea within 1-2 hours of taking your pill is the most problematic, as your body may not have absorbed any hormones yet.
  • Diarrhea 2-4 hours after taking your pill means some absorption has likely occurred, but hormone levels may still be lowered.
  • Diarrhea more than 4 hours after taking your pill is less concerning, as your body has likely absorbed most of the hormones by that point.

So episodes of diarrhea or vomiting that occur closer to the time you take your pill present the greatest risk of reducing the effectiveness of your birth control.

How long does the birth control risk window last?

After having diarrhea, the window of reduced birth control effectiveness can last up to 48 hours after symptoms resolve. This 48 hour window is considered the “risk window” where your chances of getting pregnant are higher if you have unprotected sex.

So it’s recommended that you use a backup form of birth control, like condoms, if you have sex during this 48 hour risk window after experiencing diarrhea or vomiting.

What should you do if you have diarrhea while on the pill?

Here are some tips if you experience diarrhea while taking birth control pills:

  • Take your next pill at the normal scheduled time. Do not double up doses.
  • Use a backup method like condoms during sex for 48 hours after diarrhea stops.
  • If you vomit within 2 hours of taking your pill, take another pill from your pack.
  • If diarrhea lasts more than 24 hours, consult your doctor about alternative contraception until stools normalize.
  • Check with your doctor if you have severe vomiting or diarrhea that lasts more than 2 days.

Who is most at risk?

Those at highest risk of birth control failure due to diarrhea include:

  • Women who experience vomiting along with diarrhea.
  • Those with diarrhea from food poisoning, stomach flu, Crohn’s disease, ulcerative colitis, or infection.
  • Women who have multiple watery stools shortly after taking their pill.
  • Those unable to keep fluids down due to vomiting.

In these cases, it’s especially important to follow the recommendations above to maintain birth control effectiveness.

How effective is birth control with diarrhea?

The effectiveness of birth control pills decreases with diarrhea and vomiting. Here is how much the effectiveness can drop:

  • With perfect pill use, effectiveness drops from 99% to about 91% if vomiting or diarrhea occurs.
  • With typical pill use, effectiveness drops from 91% to about 75% if vomiting or diarrhea occurs.

So while birth control is still somewhat effective, your chances of getting pregnant increase several times higher than normal when diarrhea is involved.

Does diarrhea affect other birth control methods?

Diarrhea can also interfere with other birth control methods:

  • Mini-pill – Vomiting or diarrhea within 3 hours can reduce effectiveness. Use backup method for 48 hours.
  • Patch – Diarrhea and vomiting can decrease absorption from the patch. If it falls off, replace it with a new patch.
  • Ring – Severe vomiting and diarrhea may lead to the ring being expelled from the vagina. Reinsert the ring as soon as possible.
  • IUD – The IUD’s effectiveness is unlikely to be impacted by diarrhea.
  • Implant – Nausea is possible when the implant is inserted, but diarrhea does not affect the implant’s effectiveness.

When should you contact your doctor?

Contact your doctor if:

  • You have severe or persistent vomiting or diarrhea that lasts more than 2 days.
  • Your birth control pills are not staying down.
  • You’ve had unprotected sex during the 48 hour “risk window.”
  • Your symptoms do not improve after taking anti-diarrhea medication.
  • You think you may be pregnant.

Your doctor can provide tailored guidance on maintaining contraceptive effectiveness when dealing with diarrhea.

Key Takeaways

  • Diarrhea can speed up passage of pill hormones through the body, lowering absorption and effectiveness.
  • Use backup birth control like condoms during the 48 hour window after diarrhea stops.
  • Vomiting along with diarrhea further reduces birth control effectiveness.
  • Contact your doctor if diarrhea lasts more than 2 days or you think you may be pregnant.
  • With diarrhea, birth control effectiveness can drop from over 90% down to around 75%.


In summary, diarrhea can substantially impact the effectiveness of birth control pills and some other contraceptive methods. The key is to be aware of the increased pregnancy risk that diarrhea causes so you can take steps to prevent birth control failure. Using a backup method and contacting your doctor if diarrhea is severe or persists can help ensure you maintain protection against unwanted pregnancy.

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