The "drug" that Lewis sings about is women. They don't have the side effects most drugs do - dry mouth, red eyes, make your face break out, etc.
Lewis sued Ray Parker Jr. for stealing this on his hit "Ghostbusters." They settled out of court, but Parker sued Lewis in 2001 after Lewis revealed in a VH1 Behind The Music special that Parker paid him as part of the agreement.
This song was one of 4 US Top-40 hits from the album. The others were "Heart And Soul," "If This Is It," and "Walking On A Thin Line." The hits were released over the course of a year in 1983 and 1984, and led to his biggest hit, "Power Of Love," in 1985.
The movie Back To The Future went into production with this as a temporary track until a more suitable song was written, which turned out to be "Power Of Love." This did make the movie - it is in a scene where Marty McFly listens to it as his alarm clock goes off.
Weird Al Yankovic did a parody of this song titled "I Want A New Duck." When Al appeared in a parody of American Psycho, he is killed by Huey Lewis, who exclaims: "try parodying one of my songs now, you stupid bastard." (thanks, Matthew - Milford, MA)