I have a nightmare story from Mirena. I had it placed twice, the first one for five years (what it was approved for at the time) and the second for about 2 and a half years. It can cause a lot of hormonal issues in some people as well as causing lack of impulse control with any addictive substances (for me, sugar and salt).
So if you already struggle to eat responsibly it may cause a LOT of weight gain.
For me this was enough weight gain to need my gallbladder removed about 6 months before the second one was placed. I found out in September 2015 when I was in the hospital for a life-threatening case of pneumonia (like fever between 104 - 105 F/40-41 C in a 27 year-old). So I needed to finish coughing to have the surgery in October 2015.
I lost about 60 lbs (25 kilos) in the six months prior to having my second Mirena placed in February 2016, so I learned the hard way that if you lose a lot of weight and then get it replaced, it can cause really severe mood symptoms. I also moved abroad that month, started eating less/IF alternating binging, walking more and smoking more (not good. Don't smoke with any HBC; it's a bad idea in general too.) I quit smoking so I could walk up the stairs from the metros in my new city... lots of stairs lots of times per week, so I lost a lot of weight doing this too... and the speedy weight loss worsened the mood symptoms caused by the artificial progesterone.
Having the second one removed caused PCOS, months of mood swings, and happened concurrently with a cervical cancer scare that took 14 months to resolve (which is not usually the case for borderline changes in cervical tissue, but I was living in a country with abysmal reproductive healthcare where abortion is illegal and even finding a pharmacist that stocks Plan B can be an ordeal; once I had to walk to 5 pharmacies just to find it, they kept telling me they never carry it until I yelled at the 4th one that I knew it was illegal and I would report all the pharmacies in that municipality and write about my ordeal in the local facebook groups if she did not point me in the right direction where I could get it immediately. I was living there with my longterm, monogamous partner whom I was married to and ended up getting a vasectomy at the time.) This was 2 years ago now.
My partner and I ended up moving to a country that's usually got really good reproductive healthcare after the first 7 months of getting it removedand I say "usually" because many health resources have been redirected for coronavirus right now, BUT even during the social distancing I was approved for a laparoscopic tubal ligation in April of this year and had a copper IUD placed at the same time as a stopgap measure.
My partner broke up with me just over a year ago now... all this stress and the moods were too much to handle and there were a lot of other factors playing into how the relationship fell apart that have since been resolved on my end. Many didn't have much to do with Mirena but it did change how I reacted and responded to a lot of things and how emotionally over-the-top I reacted in many situations and it took months to resolve after having it removed in June 2018.
So I know Mirena seems great and often feels great but it can change your mind and behavior very quickly and you can't always realize it, especially if you have major life changes affecting your mental health when it's placed: for the first one, my family (Catholic, patriarchal, too traditional) was having a hard time coping with my coming out as queer and being really emotionally abusive about that too, but did support my having birth control in the hopes I'd have a male partner again soon (which my dad, who is a GP, has since admitted!)
Be careful. Consult your doctor about the moods and don't write them off as no big deal.