The rumor came about because the drink included propylene glycol, a synthetic liquid also found in antifreeze. Propylene glycol appears in many processed foods.
Propylene glycol is a useful synthetic substance for absorbing water and enhancing flavor. It's also a preservative and emulsifying agent that can thicken foods.
The USDA considers any product with less than 50 grams of propylene glycol per kilogram safe. Fireball's formula contained less than an eighth of that amount.
However, European standards are more stringent, and it was banned in Norway, Sweden, and Finland in 2014. Since then, the company has removed propylene glycol from the formula.