When using Mirena, about 0.1% of the maternal dose of levonorgestrel can be transferred via milk to the nursed infant.[30] A six-year study of breastfed infants whose mothers used a levonorgestrel-only method of birth control found the infants had increased risk of respiratory infections and eye infections, though a lower risk of neurological conditions, compared to infants whose mothers used a copper IUD.[31] No longer-term studies have been performed to assess the long-term effects on infants of levonorgestrel in breast milk.