Heroin is reportedly one of the most addicting substances in the world.
That's according to a popular study from 2007 published in The Lancet. It examined dependence and other factors.
Heroin, an opiate, raises the level of dopamine in the brain.
Dopamine is one of four chemicals responsible for happiness.
Over half a million Americans tried heroin in 2013 — a 150 percent increase in about six years.
In 2013, more than 8,000 people in the U.S. died from heroin overdoses.
Cocaine, street methadone, barbiturates and alcohol rank high with causing dependence, right after heroin.
This video includes clips from the National Institute on Drug Abuse and images from Getty Images.