OCT 05, 2022 8:53 AM PDT

Emergency Contraception Pills are Significantly Less Effective in People Who Are Overweight

WRITTEN BY: Zoe Michaud

Emergency contraception pills can be used to prevent pregnancy within 72 hours of having unprotected sex. When taken as directed, emergency contraception pills can reduce the risk of pregnancy by up to 87%. 

Emergency contraception pills (which go by brand names such as Plan B One Step, Take Action, and My Way) contain a hormone called levonorgestrel, which prevents pregnancy by preventing or delaying ovulation. Levonorgestrel only affects ovulation and will not end a pregnancy in a person who is already pregnant. These medications are generally safe but can cause side effects including nausea, abdominal pain, fatigue, and headache. 

Meta-analysis studies have shown that emergency contraception pills are ineffective in women above a certain body mass index (BMI) and weight. At the standard dose (1.5mg of levonorgestrel), emergency contraception pills begin to lose potency in women weighing 165 pounds. In women weighing 175 pounds or more, emergency contraception pills typically do not work at all. 

Since the average weight of a woman in the United States is around 166 pounds, this information is critical in informing individual and public health decisions. The copper intrauterine device (IUD) is an effective form of emergency contraception for women regardless of weight or BMI. The copper IUD successfully prevents pregnancy 95% of the time when placed within 72 hours of having unprotected sex. 

The copper IUD does not contain hormones at all. Instead, the copper IUD acts as a barrier after it is inserted into the uterus. Copper is naturally toxic to sperm and eggs, preventing pregnancy. Once the copper IUD is in place, it can remain in the uterus as a birth control device for up to 10 years. After the copper IUD is removed, fertility returns to normal within a week. 

Sources: Planned Parenthood, Contraception, Mayo Clinic

About the Author
Biology
Zoe (she/her) is a science writer and a scientist working in genomics. She received her B.S. from the University of Connecticut with a focus in Evolutionary Biology. At Labroots, she focuses on writing scientific content related to clinical research and diagnostics.
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