The Terrible Truth (1951)

A Juvenile Court judge is at a loss to understand why so many of America's youths are marijuana addicts, so he decides to investigate on his own. He visits Phyllis, a high school senior and former heroin junkie, who tells him about the horrible effects heroin has had on her. She managed to overcome her addiction to marijuana and heroin, but in the process ruined her hair. This leads the judge to the logical conclusion that the drug problem in the U.S. was introduced by the godless Soviet Communists in an effort to "undermine morale" and that the way to stop the drug epidemic was to "use common sense" (an earlier version, apparently, of the Reagan-era "Just Say No!" campaign, and which had pretty much the same effect--i.e., none).