When I drink dr. Pepper I get a slight asthma attack, why is this?

Sodium benzoate. Can trigger an asthma attack, and the pepper's got it.
Sodium benzoate. Dr. Pepper contains sodium benzoate, a flavor preservative that triggers chest tightness in some folks with asthma. Sodium benzoate is used in many soft drinks & other foods. Its possible that there's more in dr. Pepper than other drinks or perhaps you're just a "pepper". Effervescence may play a role. Try pouring the soda back & forth between 2 glasses three or four times before drinking.