The only 100% effective method—the one that guarantees with absolute certainty that you won't become pregnant—is abstinence from sexual activity. Following closely behind in effectiveness is the birth control pill, which is touted as being 98-99% effective for women who take their pills as instructed. The numbers aren’t so good for those who miss pills or take certain medications, either prescribed or over the counter—for those unfortunate women, the effectiveness decreases which can result in a surprise pregnancy. Let’s get into the details of ‘The Pill’.

Effectiveness of Birth Control Pills When Taken as Directed

Birth control pills are made of hormones, chemicals which affect how our bodies work and react. There are two types of birth control pills: those which contain progesterone only, and those which contain progestin and estrogen. The second type are referred to as combination pills; the majority of women are prescribed this type.

When you’re looking at how well the birth control pill works it's vital to note that taking them every day is the key to their being most effective at preventing a “surprise” for you and your partner down the road—the surprise being an unwanted pregnancy after that night of unbridled passion and sex when you thought you had all the bases covered, so to speak.

For combination pills, it is important that you take the pill each day; for progestin-only pills, it is important that you take the pill the same time each day to prevent birth control failure. It's long been advised that a woman take her birth control pill at the same time each day, coupling it with a routine daily activity, such as always taking it before breakfast, or always taking it after brushing one's teeth as this cements the two activities together, making it less likely you'll forget to take your daily pill.

Seven Day “Waiting” Period

You do know that when you first begin taking the oral contraceptive pill (OCP), it is not effective until after the 7th consecutive day of pill-taking, right? Health care providers will have explained this during your visit, but some women don't catch all the facts. This is an important detail if you engage in sexual activity during that seven day window.

Influences on the Effectiveness of the Pill

Other things which can decrease the effectiveness of your OCP include you weight; very overweight women may experience less effectiveness of the pill. The antibiotic rifampin (prescription) is known to decrease the pill's effectiveness. Many antibiotics have the same consequences by killing bacteria that normally increase the concentration the hormones in the OCP reach in your body.

St. John's Wort (over-the counter) also interferes with the OCP, as do some anti-seizure medications (prescription), certain HIV meds (prescription), and certain oral medications taken for yeast infections (prescription).

This is an area where you will need to be diligent, as the prescriber of your OCP is likely to be a different health care provider than the one who would be prescribing most of these other medications, and of course, St. John's Wort, which you would be buying over-the-counter. When in doubt, ask a pharmacist about potential interactions or decreased effectiveness of your OCP with your other medications.

“Vomiting and diarrhea may also keep the pill from working”. How much vomiting and diarrhea? There is no hard and fast rule, so it is best to take additional precautions against pregnancy, your Plan B, if you've experienced these symptoms.

Skipping or Missing a Pill or Pills

This is an area where being forewarned enables you to be forearmed—in other words, you can formulate a plan ahead of time should you happen to skip a pill or pills. It happens to many women at some time, so it's best not to believe it couldn't happen to you.

First, you need to know whether you are taking a combination pill (estrogen and progestin) or a progestin-only pill. The likelihood of failure of the pill to be effective is going to be based on how many days you have missed with the combination pill; with the progestin-only pill, it is based on the number of hours.

In both cases, take the pill as soon as you remember. Take the next pill at the usual scheduled time. Refer to this table for specific information as to how many pills you missed in what part of the pack for combination pills and for more specific information about the progestin-only pill.

Plan B

In life it is often wise to have a Plan B; in preventing a surprise pregnancy its importance is directly proportional to how badly you want to remain un-pregnant. Your Plan B may include use of condoms, female condom, diaphragm, sponge, or emergency contraception such as the morning-after pill (In fact often called: Plan B)