The purpose of the study is to investigate if the study drugs (tranexamic acid or mefenamic acid) can control irregular bleeding during the first 3 months of using Mirena. The study drugs tested are tested against placebo ("dummy medication not containing any active drug"). Treatment period is followed by a one-month period when study drugs are not taken but Mirena use is continued.

Inclusion Criteria:

• Signed and dated informed consent
• Healthy female subjects requesting contraception
• Age: 18 - 45 years inclusive
• Successful interval insertion of MIRENA
• History of regular cyclic menstrual periods
• Normal or clinically insignificant cervical smear not requiring further follow up

Exclusion Criteria:

• Pregnancy or lactation
• Climacteric symptoms prior to the screening visit
• Known or suspected clinically significant ovarian cysts, endometrial polyps, fibroids, or other genital organ pathology, that, in the opinion of the investigator, may interfere with the assessment of the bleeding profile during the study
• Undiagnosed abnormal genital bleeding
• Current or history of thrombembolic disease, or established risk factors for venous thromboembolism
• Current migraine, focal migraine with asymmetrical visual loss or other symptoms indicating transient cerebral ischemia, or exceptionally severe headaches
• Hypersensitivity to any ingredient of the investigational medicinal products or the non-investigational medicinal product
• Daily or frequent use of a nonsteroidal anti-inflammatory drug (NSAIDs) for any condition
• Not willing to use nonsteroidal anti-inflammatory drug (NSAIDs) medication as pain medication during the double blind treatment period