Hi L.,
As with any medication, each person has the possibility of reacting differently. That's definitely been the case in the responses, so I'll add mine. I used Mirena for three years, had no periods at all and gained 40lbs. I had it removed and got pregnant three months later, so I'm not sure how my weight would have changed over a longer period of time.
This is from the FDA website regarding Mirena.
"Adverse reactions reported by 5% or more of clinical trial subjects include:
Abdominal/pelvic pain
Vaginal discharge
Nausea
Headache
Nervousness
Vulvovaginitis
Dysmenorrhea
Back pain
***Weight increase
Breast pain/tenderness
Acne
Decreased libido
Depressed mood
Cervicitis/Papanicolaou smear normal, class II
Hypertension
Other relevant reported adverse reactions occurring in less than 5% of subjects include:
migraine
vomiting
anemia
dyspareunia
alopecia
eczema
pruritis
rash
urticaria
abdominal distension
altered mood
hirsutism
edema."
Only you can make the right choice for you, but it's easier when you know what you're battling! Good Luck!