What are the signs that Plan B didn't work?

If you have had to take Plan B as an emergency contraceptive, you may be wondering if it has worked. While research shows it has an efficacy rate of 89% when used correctly, how easy is it to tell if it has worked for you?

The easiest way to know that it hasn’t worked are a missed period and a positive pregnancy test. Early signs of pregnancy are not a reliable way to tell if Plan B has worked or not – unless they correspond with a positive pregnancy test. This is because many women do not experience ANY early symptoms of pregnancy, such as:

• Nausea, or feeling sick, with or without vomiting
• An increase in vaginal discharge
• Bloating
• Dizziness
• Increased heart rate
• Increased urination
• Light spotting
• Mood swings
• Motion sickness
• Raised body temperature
• Sensitivity to smell
• Taste changes
• Tender, swollen, tingling, or aching breasts
• Tiredness
• Weight gain.

These early symptoms of pregnancy are also very vague - and could be because of another reason, not because you are pregnant.

How early can you tell Plan B hasn’t worked?

The earliest a pregnancy test can pick up a pregnancy is 10 days after conception. But these can often be negative, even if you are pregnant, so it is best to wait until you’ve missed a period to take a test.

You cannot reliably determine that Plan B hasn’t worked until you take a pregnancy test.

Related questions

• Can Plan B make your period late or cause bleeding?
• Can you have sex after taking Plan B?
• Can you drink alcohol after taking Plan B?

How common is it for Plan B to fail?

The earlier Plan B is used, the more effective it is. Plan B is highly effective when used within 72 hours. It works by delaying ovulation, which can prevent sperm from fertilizing an egg.

Pregnancy does not always happen following sex, even if no contraception is used. Research has shown that for every 100 people with a uterus that have sex without any contraception, 18 will get pregnant.

The chances of becoming pregnant increase if sex is timed so that it happens when a person is most likely to be ovulating. This increases the pregnancy rate to 20 to 28% (meaning that for every 100 people with a uterus that have sex without any contraception, 20 to 28 will get pregnant).

Plan B makes pregnancy less likely. Research has found that only 16 out of 1000 (2.4%) women who sought emergency birth control ended up pregnant. In tests by the manufacturer, Plan B One-Step’s tests showed it was effective at preventing 84% of pregnancies.

Why would Plan B fail?

Plan B can fail if a person:

• Takes Plan B more than 72 hours after intercourse
• Vomits within 2 hours of taking Plan B
• Has a larger body mass and does not take the dosage that is right for them
• Ovulates before taking Plan B
• Takes other medications, such as St John’s Wort, or seizure medications that can interact with Plan B making it less effective
• Is already pregnant before taking Plan B (Plan B will not affect a pregnancy that has already happened).