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When you take the pill every single day, it’s great at preventing pregnancy. But missing pills, taking certain medications, and other things may make it not work as well.

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How effective is the birth control pill?

When used perfectly, the pill is 99% effective. But when it comes to real life, the pill is about 91% effective because it can be hard to be perfect. So in reality, 9 out of 100 pill users get pregnant each year.

The better you are about taking your pill every day and starting your pill packs on time, the better it will work. But there’s a very small chance that you could still get pregnant, even if you always take your pills correctly.

If effectiveness is the most important thing to you when picking what birth control to use, you might want to check out IUDs and the implant. They’re the most effective kinds of birth control.  But if you decide the pill is right for you, make sure you take them on time for the best effectiveness.

How long do birth control pills take to work?

It depends on when you start taking them. You can take your first birth control pill any day of the month, but you may need a backup birth control method (like condoms) for the first 7 days.

Combination Pills (COCs)

  • If you start combination pills within 5 days after your period starts, you’ll be protected from pregnancy right away. For example, if you get your period Monday morning, you can start the pill anytime until Saturday morning and be protected from pregnancy right away.

  • If you start combination pills any other time, you’ll be protected from pregnancy after 7 days of taking the pill. Use another method of birth control — like a condom or female condom — if you have vaginal sex during the first week you’re on the pill.

Progestin-Only Pills (Mini Pills)

You can start progestin-only pills at any time. You’ll be protected from pregnancy after 48 hours (2 days). So use another method of birth control (like condoms) if you have vaginal sex during the first 48 hours.

Your nurse or doctor can help you figure out the best time to start your birth control pills, and when they’ll start working for you.

What lowers the pill’s effectiveness?

The main thing that makes the pill not work is not taking it every day. But other things, like having vomiting or diarrhea for more than 48 hours may reduce how well the pill prevents pregnancy. The pill may be a little less effective for very overweight women. Some medicines or supplements can also make it not work as well:

• The antibiotic Rifampin (other antibiotics do not make the pill less effective)

• The antifungal Griseofulvin (other antifungals do not make the pill less effective)

• Certain HIV medicines

• Certain anti-seizure medicines

• The herb St. John’s Wort

If you’re taking any of these, use condoms as a backup method. Switch to a different method of birth control if you’ll be on them for a long time.

Your nurse or doctor can help you decide if there’s any reason the birth control pill won’t work well for you.

The Pill

  • 91% effective

  • Costs up to $50, but can be $0

  • Prescription required

  • Take once a day

The pill doesn’t protect you from  STDs. Use a condom with your pill to help stop pregnancy and STDs.
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