What causes the birth control pill to fail?

No birth control is 100% effective. I repeat, NO BIRTH CONTROL IS 100% EFFECTIVE. The pill is close, at 99% with perfect use, but only about 92% with "typical use." Even if you use your pill absolutely as directed, you could still become pregnant. Plus, most women don’t use it perfectly. What does that mean? Here’s a list of things that can go wrong:

• You miss a pill(s), or were late taking the pill
• You aren't taking your pill at the same time every day. If you aren’t taking the pill at the same time every day, it increases the chances that you’ll ovulate, and if you miss one pill or more, you’re further increasing the chances that you’ll ovulate, which means you could get pregnant. Check your pill package for instructions on what to do when you miss one, but to be on the safe side, use a backup method for at least one week.

• Illness. If you vomit or have diarrhea, particularly within a few hours of taking your pill, it may not be absorbed into your system. That means you've effectively missed a pill, which could leave you unprotected.
• Medications. There are a number of medications that can decrease the effectiveness of the birth control pill. If you take any of the following in a month, you should be using a backup method. If you’re going to take any medication at all you should ask a pharmacist if it could interfere with your birth control, especially if it falls into any of these categories:
  
  -Seizure medications
  -Sleeping pills
  -Antibiotics
  -Antihistamines and cold medications
  -Antacids
  -Some herbal remedies
  
  
• Incorrect use. If you don't follow the instructions on your pill package, the pill may not work as well. A few common mistakes to be avoided are:
  
  -Do not wait for your period to end before you start a new package. Start on the pill schedule, which is after the last sugar pill if using a 28-day pack, or after seven days off if using a 21-day pack.
  -Do not rely on the pill for the first month you’re on it - use backup. If you switch types of pills, wait a month to be on the safe side.
  -Do not assume you can't get pregnant. It does not take time to get the pill out of your system, which means that you could get pregnant as soon as you stop taking it.