Complex sentence: Mirena releases the hormone at an initial rate of 20 micrograms per day and declines to a rate of 14 micrograms after 5 years, which is still in the range of clinical effectiveness.
Split into simpler sentences: Mirena releases the drug at an initial rate of 20 micrograms per day. This declines to a rate of 14 micrograms per day after 5 years, which is still in the range of clinical effectiveness.