Mirena contains levonorgestrel, a synthetic form of progesterone. The device holds 52 mg of this hormone and releases it slowly into the uterus over several years, starting at about 20 micrograms per day. That local delivery is what makes Mirena different from birth control pills or implants: most of the hormone stays in the reproductive tract rather than circulating through your entire body.
What Levonorgestrel Does in the Body
Levonorgestrel is a progestin, meaning it mimics the natural hormone progesterone that your body produces after ovulation. It’s the same hormone found in many birth control pills and the Plan B emergency contraceptive, but Mirena delivers it in much smaller daily amounts and directly where it’s needed.
Inside the uterus, levonorgestrel works through several mechanisms at once. It thickens the mucus in your cervix, creating a barrier that sperm struggle to pass through. It thins the uterine lining, making it less likely that a fertilized egg could implant. It also interferes with sperm movement and survival. In some women, it partially suppresses ovulation, though this isn’t the primary way Mirena prevents pregnancy.
Because the hormone is released directly into the uterus, blood levels of levonorgestrel in Mirena users are significantly lower than in women taking a daily progestin pill. This localized approach is why many women tolerate Mirena well even if they’ve had trouble with systemic hormonal birth control in the past.
How the Hormone Release Changes Over Time
Mirena doesn’t release the same amount of hormone throughout its lifespan. It starts at roughly 20 micrograms per day and gradually tapers as the reservoir depletes. After five years, the daily release drops to about 10 micrograms per day. This declining curve is important for understanding both the device’s effectiveness and its side effects: many women notice that hormonal side effects, if they have any, tend to ease in the first year or two as the release rate settles.
Mirena is FDA-approved for pregnancy prevention for up to 8 years. For treating heavy menstrual bleeding, though, the approved duration is only 5 years, likely because the lower hormone output in later years may not be enough to keep controlling heavy periods. If you’re using Mirena specifically for heavy bleeding, your provider will typically recommend replacing it after year five.
Effects on Your Period
The thinning of the uterine lining is what gives Mirena its well-known effect on periods. Clinical data shows an 85% reduction in menstrual blood loss within three months of insertion, increasing to a 97% reduction by 12 months. About 9 out of 10 women with heavy periods see a significant improvement. Many Mirena users eventually stop having periods altogether, which is a normal and expected result of the hormone thinning the uterine lining, not a sign that anything is wrong.
In the first three to six months, irregular spotting and breakthrough bleeding are common as your body adjusts to the constant low-dose progesterone exposure. This transition period can be frustrating, but it typically resolves on its own.
Side Effects Linked to the Hormone
Even though Mirena acts locally, small amounts of levonorgestrel do enter the bloodstream. Some women experience hormone-related side effects including headaches, acne, breast tenderness, mood changes, and decreased sex drive. These tend to be milder than with oral contraceptives because the systemic dose is so much lower, but they’re real and worth tracking if you notice changes after insertion.
Ovarian cysts are another hormone-related effect. Levonorgestrel can alter the normal ovulation cycle just enough that follicles sometimes enlarge into small cysts. Most of these are harmless and resolve without treatment within a couple of months, though they occasionally cause pelvic pain.
How Mirena Compares to Other Hormonal IUDs
All hormonal IUDs currently available use levonorgestrel. The difference is how much. Here’s how they compare:
- Mirena: 52 mg total, releasing about 20 mcg/day initially, approved for up to 8 years
- Liletta: 52 mg total, releasing about 18.6 mcg/day initially, with a similar profile to Mirena
- Kyleena: 19.5 mg total, releasing about 17.5 mcg/day initially, dropping to 7.4 mcg/day after 5 years
- Skyla: 13.5 mg total, releasing about 14 mcg/day initially, dropping to 5 mcg/day after 3 years
Mirena and Liletta carry the highest hormone load, which is why they’re the most effective options for treating heavy menstrual bleeding. Kyleena and Skyla, with their lower doses, are physically smaller and sometimes preferred for women who haven’t had children, though all four are approved regardless of whether you’ve given birth. The trade-off with lower-dose IUDs is that they’re less likely to stop your period entirely and may be slightly less effective at reducing heavy bleeding.
If you’re choosing between these options, the core question is what you need the device to do. For contraception alone, all four are highly effective. For period management, Mirena’s higher levonorgestrel dose gives it an edge.