When your initial choice of contraception fails, Plan B is expected to be work…however, a ‘plan c’ may need to be devised for women weighing 165 pounds or more.

New research suggests that the most popular emergency contraceptive, which is a morning-after pill that uses the hormone levonorgestrel that prevents ovulation, is less effective for women weighing more than 165 pounds. However, one should bear in mind that the average weight of the American woman is 162 pounds during her 20s. During her 30s, the American woman weighs about 169 pounds.

Likewise, European-made Norlevo, a morning-after pill, loses effectiveness for those weighing 165 pounds. Furthermore, it’s isn’t at all effective for women weigh more than 175 pounds. Despite the fact Norlevo is chemically identical to the U.S.-made Plan B, the Food and Drug Administration has insisted the data was inconclusive, warranting no warning here in the U.S.

Alison Edelman, a professor in the Department of Obstetrics and Gynecology at the Oregon Health and Science University, lead a new study that analyzed the effectiveness of levonorgestrel-based contraceptive among women with a BMI within the normal range, comparing that to the BMI of women considered obese. It was discovered that it would take twice the dosage of Plan B to increase an obese woman’s hormone level of women weighing less than 160 pounds. However, taking two pills for the purposes of emergency contraception isn’t recommended yet, according to Elizabeth Dawes Gay of Women’s Health Magazine. The ovulation-inhibiting hormone could be doubled, but more test would need to be done before learning what how those hormones may affect a woman’s ovaries.

Plan B is emergency contraception that’s generally effective and safe, and it’s a backup plan that helps to prevent pregnancy with 72 hours following birth control failure, preventing 7 out of 8 potential pregnancies. Side effects of the pill are tiredness; a headache; dizziness; breast tenderness; vomiting; lower abdominal pain/cramps; and lighter, heavier, later or early periods. The FDA-approved emergency contraception is accessible over the counter, and should not be considered if you’re already pregnantΩ and shouldn’t be used as birth control.