Complex sentence: In 2002, the United States Department of Health and Human Services estimated 12 million people, age 12 and over, had used methamphetamine--600,000 of which claimed to be current users--with a growth rate of approximately 300,000 new users per year.
Split into simpler sentences: In 2002, the U.S. Department of Health and Human Services released a report estimating 12 million people, age 12 and over, had used methamphetamine--600,000 of which claimed to be current users. They further estimated a growth rate of approximately 300,000 new users per year.