According to the Substance Abuse and Mental Health Services Administration's National Survey on Drug Use and Health, approximately 20 percent of Americans experienced mental illness in the last year. And about 5 percent of the U.S. adult population, or 11.4 million people, suffered from a serious mental illness, which is defined as a mental illness that resulted in serious impairment and severely limited at least one major life activity.

Among those suffering from serious mental illness, the rate of substance abuse or dependence was over 25 percent, compared with 6.1 percent of those who had not suffered from mental illness.

In general, the study defined mental illness as having had a diagnosable mental, emotional or behavioral disorder on the basis of certain criteria, according to a Medscape article.

The rate of mental illness was higher among those ages 18 to 25, at nearly 30 percent. That's compared to 14.3 percent of those ages 50 and older who suffered from a mental illness in the last year. Women were more likely than men to have suffered mental illness at 23 percent, compared to 16.8 percent for men.

The study also found that about 8.7 million Americans considered suicide in the last year. Of those, 2.5 had made plans to commit suicide, and 1.1 million actually attempted suicide.

According to SAMHSA, the overall economic impact of mental illness was about $300 billion back in 2002. An administrator for SAMHSA says that people would greatly benefit from access to mental health services. She notes that people are often able to manage mental illnesses successfully, and many of them do recover.

Source: Medscape, "High prevalence of mental illness among Americans," Caroline Cassels, Jan. 26, 2012