Can metronidazole inhibit mirena (levonorgestrel) effectiveness?

Yes: Most antibiotics "compete" with other drugs for protein binding sites and may reduce effectiveness of other rx's.Your pharmacist is an expert on these interactions.

No: The Mirena (levonorgestrel) IUD is one of the BEST forms of birth control available, it lasts 5 years, is ~99.5% effective, and thins the uterine lining so that you get lighter periods. 20% of women will stop having periods after 1 year and that's perfectly healthy. Effects in the uterus and so it won't affect or be affected by other medicadtion; very safe method; biggest risk after insertion is expulsion (1-2%).