Why does soda make your mouth worse after eating something spicy?

Carbonation creates a bigger shock. The cold and the carbonation in the soda acts as an irritant to the mouth. This can add even more fire to an already uncomfortable situation.

Soda washes the capsaicin around the mouth. The sweeteners or flavors in the soda can break down the oily molecules in peppers that contain the capsaicin, which can then end up coating even more of the mouth and throat.

Soda reduces saliva. Saliva is an important tool in breaking down food and protecting your mouth from potential insults, such as spice and heat. Soda decreases saliva production, which makes it more difficult for the mouth to recover.