Yes. Mirena (levonorgestrel) is a Progesterone (only) containing intrauterine contraceptive. The number one side effect of Progesterone only birth control is unpredictable vaginal bleeding. This occurs in up to 80% of patients. The majority of those patients will eventually experience a decrease in the bleeding, but that can take 6 or 8 months - or longer. Other side effects are weight gain and depression.