I’ve read the effectiveness is over 99%, specially with hormonal IUDs like Kyleena. What happens to the rest of the people? Are they just built different? How is it possible that the egg still gets fertilised and implanted even with all those hormones making the uterus lining thin for implantation and the cervical mucus too thick for the sperm to get to the egg in the first place?
In: Biology
Because the human body (and really a broad part of the animal kingdom in general) is an incredibly complex biological machine with a lot of variation not just between individuals but within one individual over time. Our ability to manipulate how the body works via drugs is, in many ways, very limited.
Also effectiveness of 99% etc. doesn’t usually mean “Works 100% for some people 0% for others”. While the effectiveness CAN be higher or lower from one individual to another, it can also vary for an individual, again, because humans are complex biological machines. The foods you eat, the time of year, how much sleep you got, whether you are exercising or not, etc. all of these things (and more) can affect how your body works and as a result can change whether a particular drug is effective enough for its purpose.
Most forms of medication are not tailored for the individual, they are set at a level based on what will be effective for a broad group of people. It’s expensive and/or impractical to customize the medication for just you.
Combine all those factors (variation between individuals, variation within an individual, imprecise medication dosage, etc.) and you come up with a good but not perfect system
Most IUD failures are a result of the IUD getting dislodged out of place.
The most recent number I heard was 99.8% effective over 1 year (2 pregnancies in 1000 patients).
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