The rumors about Yellow Dye No. 5 gained a bit of traction in 2009. Scientists at the University of Oran, Algeria found that when mice drank an excessive amount of the yellow dye, their sperm count decreased and abnormalities increased (via Vice). However, this study has never been replicated with human subjects. The FDA reassessed tartrazine in 2009 and found no indications of adverse effects on reproduction (per Medium). Additionally, Mountain Dew is not the only soda or product that contains Yellow No.5, as it can also be found in drinks like Gatorade, Fanta Pineapple, and Surge.
Tartrazine may have been the main target in the infertility rumor mill, but rumors also spread about the soda’s caffeine content having negative effects. Mountain Dew does contain slightly more caffeine than other sodas like Coca-Cola, which made it an easy target for these rumors. According to Medical News Today, however, caffeine is unlikely to affect fertility. They reference a study by the Nutrition Journal, which analyzed 28 sperm measurement studies and concluded that there is no consistent evidence of caffeine affecting sperm count or infertility.
So remember, if you “Dew the Dew” before you do the deed, you still need to use protection.