Women have complained of significant weight variations while on Mirena’s intrauterine devices (IUD), industry experts reveal, quoting numerous consumer reports and medical sources. This birth control system has been listed as causing significant weight changes that affect a percentage of the women taking it. The changes in weight result from treatment differ in each person, with weight gains being more common than weight loss, but this side effect is only one of other serious side effects linked with this type of hormonal birth control system.