As of 2021, more than 36 million people were estimated to be experiencing substance use disorder. This condition spans a spectrum of patterned drug use that causes issues in a person’s life, with substance addiction at the more severe end. Why are some people more susceptible to addiction, and why can it be so difficult to treat? Judy Grisel takes a look at how addictive drugs affect the body. Lesson by Judy Grisel, directed by Gabriella Marsh.