New York (CNN Business) LaCroix was once the "it" brand in one of the fastest-growing parts of the beverage market. But bigger, deeper-pocketed companies have caught up, and LaCroix's popularity is fizzling.

Sales for the flavored carbonated water are "effectively in free fall," wrote Laurent Grandet, a beverage analyst for Guggenheim, in a new note. Sales have fallen more than 15% in May after tumbling nearly 7% in April, 5% in March and 6% in February, according to Guggenheim.

"The LaCroix brand has gone from bad, to worse, to disastrous in a relatively short period of time," Grandet said, citing increased competition for similar-type beverages and the "lack of meaningful or disruptive innovation" from the drink's owner, National Beverage.

National Beverage's FIZZ stock price has fallen 10% this week and 62% since September 2018.

American consumers increasingly seek out drinks that have functional elements, like nutritional value or a jolt of caffeine, according to research company Mintel. They also want healthy, low-sugar or sugar-free offerings. Sparkling water ticks a lot of those boxes.

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