Reported overdose deaths from illicit drugs rose from about 5,309 in 2000 to 21,823 in 2015. This is an increase of 311% or 4.1 times during a period when the U.S. population increased by 14%. In 2015, the majority of overdose deaths from illicit drugs were from heroin. The U.S. Centers for Disease Control and Prevention "estimates the number of heroin deaths is undercounted by as much as 30%. This is due both to variations in state reporting procedures, and because heroin metabolizes into morphine very quickly in the body, making it difficult to determine the presence of heroin." Per the U.S. Department of Homeland Security, Mexico is "the primary source of heroin for the U.S. market."