Medically reviewed by Kristin Hall, FNP
Written by Our Editorial Team
Last updated 8/09/2020
Used perfectly, the birth control pill — both combined and the progestin-only kind — are 99 percent effective at stopping you from becoming pregnant.
However, a range of factors can make birth control less effective than normal. From your use of other medications to digestive disorders, both your habits and your general health can affect the effectiveness of the birth control pill.
We’ve listed five of these factors below, along with what you can do to make sure birth control works the way it should for you.
Several medications, including antidepressants, anti-fungal medications, anti-seizure drugs and some antiretroviral drugs used to prevent and treat HIV have the potential to make the pill less effective at preventing pregnancy.
Many of these medications can significantly reduce the effectiveness of the pill, increasing your risk of becoming pregnant.
Medications known to affect the pill include antidepressants such as Zoloft (sertraline), common anti-fungal drugs such as ketoconazole and griseofulvin, the antibiotic rifampin (which is typically used to treat tuberculosis) and the narcolepsy medication Provigil (modafinil).
The HIV treatments known to affect the pill include Prezista (darunavir), Viramune (nevirapine), Sustiva (efavirenz) and Kaletra (lopinavir/ritonavir).
Finally, numerous anti-seizure drugs can affect the way your body metabolizes the hormones in birth control, making the pill less effective. These include:
If you’re prescribed any of these medications, it’s important to discuss their potential effects with your doctor before you begin using the birth control pill as effective contraception. Your doctor might recommend another form of birth control that’s less likely to be affected by your existing medications.
While the vast majority of health supplements have no effect on birth control, a select few herbs, extracts and other natural supplements can make birth control less effective. These include:
Flaxseed, which can affect digestion and make your body fail to absorb some or all of the hormones in birth control pills.
Saw palmetto, which can decrease the effects of estrogen in the body and make some combined birth control pills less effective.
Garlic pills, which are commonly used to treat cardiovascular conditions such as high blood pressure and high cholesterol.
Alfalfa, which can affect your body’s absorption of the hormones in birth control pills, making them less effective.
St. John’s wort, which can affect estrogen breakdown and cause you to experience breakthrough bleeding.
All of these supplements are best avoided if you use the pill as your only form of birth control. If you accidentally take any of the supplements listed above, it’s best to use condoms or another secondary form of birth control to reduce your risk of pregnancy.
Your body needs to fully absorb each and every birth control pill you take in order to stop you from becoming pregnant. If you have a digestive or autoimmune disorder like Crohn’s disease, Celiac disease or irritable bowel syndrome (IBS), it could make the pill less effective.
These disorders affect the way your body absorbs the hormones in birth control pills, meaning you might not receive the full dose of estrogen and progestin hormones from your pill.
If you have a digestive or autoimmune disorder that affects your ability to absorb nutrients from food, it’s important to tell your doctor when you discuss birth control. Your doctor might advise you to use a different form of birth control that’s less likely to be affected by your disorder.
As well as chronic digestive disorders, temporary digestive issues such as diarrhea also have the potential to make the birth control pill less effective.
If you use the pill as your main form of birth control, it’s important to use a second form of birth control such as condoms if you experience acute diarrhea that lasts for longer than 24 hours at a time.
If your BMI places you in the overweight or obese range, you have a higher risk of becoming pregnant while using birth control than women with a BMI in the normal range.
According to study data, half of all reproductive age women in the US are either overweight or obese. Because birth controls use a one-size-fits-all dosing model, it’s not always effective for women with larger bodies.
In a 2005 study, researchers found that women with a BMI of 27.3 or higher have a 60 percent higher risk of becoming pregnant while using the birth control pill than women with a BMI in the normal or mildly overweight range.
Women with a BMI higher than 32.2 have a more than 100 percent higher risk of becoming pregnant while using the birth control pill. In general, the effectiveness of birth control decreases as your weight and BMI increases.
This means that if you’re overweight or obese, your birth control pill might not fully protect you from the risk of becoming pregnant.
As always, the best approach is to talk to your doctor. If you’re overweight or obese, your doctor might recommend an alternative form of birth control that’s more likely to provide full protection against pregnancy.
The birth control pill works best when you take it at the same time every day, making it important to establish and stick to a routine if you use the pill as your only method of birth control.
While mistiming your pill by an hour or two won’t affect its effectiveness, it’s always best to stick to a routine. Doing this not only makes your birth control pill as effective as possible; it’s also a great way to make taking birth control part of your daily routine, helping you avoid missed pills.
If you miss a birth control pill or forget to take your pill on time, follow the instructions on the pills’ packaging. If you’ve missed several pills in a row, you’ll usually need to use condoms for at least one week while your body readjusts to the hormones and blocks ovulation again.
Timing is particularly important if you use the progestin-only mini-pill, which uses a lower dose of hormones than the combined pill. As always, consistency is best. Take your pill at a consistent time and you’ll enjoy equally consistent, predictable results.
Not sure which birth control pill is right for you? Our guide to the differences between combined and progestin-only birth control goes into detail on how the two most common types of pill differ from each other.
You can also learn more about the potential side effects of birth control in our guide to common and uncommon birth control side effects.