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The IUS is similar to the intrauterine device (IUD), but instead of releasing copper like the IUD, it releases the hormone progestogen into the womb (NHS 2018).

It thickens the cervical mucus, which makes it more difficult for sperm to move through the cervix, and thins the lining of the womb so an egg is less likely to be able to implant itself.

For some women, it can also prevent the release of an egg each month (ovulation), but most women continue to ovulate.

When looking at possible side effects of progestogen, WebMD states that:

The progesterone prescription products that have been approved by the Food and Drug Administration (FDA) are LIKELY SAFE for most people when used by mouth, applied to the skin, applied into vagina, or injected into the muscle with the advice and care of a healthcare professional. However, progesterone can cause many side effects including stomach upset, changes in appetite, weight gain, fluid retention and swelling (edema), fatigue, acne, drowsiness or insomnia, allergic skin rashes, hives, fever, headache, depression, breast discomfort or enlargement, premenstrual syndrome (PMS)-like symptoms, altered menstrual cycles, irregular bleeding, and other side effects (WebMD, n.d.)

But despite many websites indicating they are the same, Progestogen is not Progesterone (Gillson, 2007).

There is a concern that some progestogens cause adverse metabolic effects, including alterations in lipoprotein fractions and carbohydrate intolerance. The choice of progestogen, the dose, and the number of days of administration appear to be important (Stanczyk, 2007).

What about bone metabolism?

Are there any other risks involved in long term use of IUSs?

References

Gillson, G. (2007). Clarifying hormone terminology. Canadian Family Physician, 53(1), 29-30. PMCID: PMC1952548

NHS (2018). Intrauterine system (IUS) - Your contraception guide. Retrieved from: https://www.nhs.uk/conditions/contraception/ius-intrauterine-system/

Stanczyk, F. Z. (2007). Structure–function relationships, pharmacokinetics, and potency of orally and parenterally administered Progestogens. In Treatment of the Postmenopausal Woman (pp. 779-798). Academic Press. doi: 10.1016/B978-012369443-0/50067-3

WebMD (2018). Progesterone. Retrieved from: https://www.webmd.com/vitamins/ai/ingredientmono-760/progesterone

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    Progestogen itself is not a specific thing, it is a category, of which progesterone is one, as is levonorgestrel (which as far as I know is used in all the approved hormonal IUDs). So, referring to something containing progesterone as containing a progestogen is not inaccurate, just not as specific. Your question makes it sound like it's a separate, specific thing - do you really mean to ask about levonorgestrel? And specifically for IUDs? Because levonorgestrel is also an ingredient in oral birth control pills and the side effects are not necessarily the same. – Bryan Krause Jul 21 '19 at 15:27
  • I was specifically asking about IUSs. I wasn't aware that they release levonorgestrel as opposed to progesterone. – Chris Rogers Jul 21 '19 at 16:22
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    It looks like historically there was an IUD called Progestasert that released progesterone, but is no longer produced. Mirena, Skyla, Kyleena, and Liletta all use levonorgestrel - there could be other formulations available outside the US. – Bryan Krause Jul 21 '19 at 16:29
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    I'd actually never heard of an "IUS" before reading this question; it seems like in the US we call them all IUDs and then distinguish between copper IUDs vs hormonal IUDs, whereas the UK changes the acronym, in case anyone else was wondering. – Bryan Krause Jul 21 '19 at 16:32
  • To clarify, as @BryanKrause said, Levonorgestrel is a synthetic hormone in the progestogen class, made to mimic progesterone - which is the primary endogenous (produced in the body) hormone in that class. Levonorgestrel is the hormone used in all IUDs, etonorgestrel is used in Nexplanon, and there are multiple different progestogens used in oral OCPs. Also dittoing Bryan, the IUS thing tripped me up at first too! I've run into it in international research but otherwise LNG-IUD is more frequently used. – DoctorWhom Jul 25 '19 at 5:19

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