1st Variation: Mountain Dew kills your sperm
2nd Variation: Mountain Dew makes your penis shrink
Origins
I first remember hearing this legend sometime during middle school in a suburb of Dallas, TX.
Collectors comments
I decided to include this legend to add on to the several other legends about soft drinks in this particular folklore collection. Each time I heard this legend, I had the same sentiments as I did when I heard about Dr. Pepper being made with prune juice. As a rational, analytical person I assumed that kids in the Dallas area would likely hear and spread rumors such as this because Mountain Dew (produced by PepsiCo) primarily runs their operations exclusively in areas such as New York and New Jersey. Since Dr. Pepper / 7up has a near monopoly of many business in the Dallas area, I thought it was likely that Dr. Pepper would attempt to spread rumors about its competitors (much like Coca-Cola creating a legend about Dr. Pepper in California).
As it turns out, Mountain Dew is said to have some kind of adverse effect upon a males genitals (in every variation of this legend). This is often due to an ingredient called Yellow 5 in Mountain Dew. My source completely refutes the claims of this legend and went a step further, mentioning that quite a lot of common products use the ingredient Yellow 5 yet nothing is said about the adverse effects that those products may have. In short, it was concluded that Yellow 5 is harmless and does not have any known harmful effects upon humans, despite this ingredient being under scrutiny since the early 1900s.
Annotation: This legend and facts disproving it can be found at http://www.snopes.com/medical/potables/mountaindew.asp
Date of access: April 24, 2007