Dr. Pepper is made out of prune juice
Origins
Kenneth heard this urban legend from high school friends (in Monterrey Park).
Collectors comments
Since my current home is in Plano, TX (the headquarters of 7up / Dr. Pepper) I was a bit offended to hear this. I immediately told Kenneth (and all the other students who were talking with us) that this was just another stupid urban legend.
I rationalized that a rival soda company such as Coca-Cola most likely started spreading rumors about Dr. Pepper products. I was pretty sure of my conviction because I noticed that nearly all of the restaurants on and near the USC campus were dominated by Coca-Cola products. For example, nearly every restaurant in the area has a drink machine with drinks such as Sprite, Coca-Cola, Powerade, and maybe a few other assorted drinks (all of which are produced by Coca-Cola).
It is nearly the opposite anywhere in the Dallas, TX area. I remember that my high school was heavily funded by Dr. Pepper and all vending machines sold only Dr. Pepper products.
Thus, I had plenty of reasons to not believe that this was a silly rumor spread by rival soda companies to discourage people from drinking Dr. Pepper.
After a bit of research, it turns out that this urban legend is indeed false. I also discovered that people believe drinks made out of prunes are disgusting because prunes are apparently known for causing bowel movements (although I had no idea ).
Annotation: This exact urban legend can be found at:
http://www.snopes.com/business/secret/drpepper.asp
Date of access: April 20, 2007