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?
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