Coca-Cola, the iconic American soda company, is on the defensive with a new ad campaign assuring consumers that diet soda with artificial sweeteners like aspartame is safe.

"Here you have perhaps the most epic brand in the world, but they have a problem, their product is now being known as unhealthy," said branding consultant Dan Baum.

Sales are down across the board as consumers seek alternatives to sugary drinks. Even diet drinks are taking a hit.

CNN's Erin McPike reports.