The ruling, handed down July 11, is potentially significant because it weakens the ability of lawyers to create class-action suits based on false advertising claims.

The court wrote that in the past, class-action status in California had been “abused” by plaintiffs. Law firms could find one person who had allegedly been misled into buying a product and then sue as a class on behalf of any customer who bought the product.

In the ruling, the court said that plaintiffs now had to show much more specifically that everyone who had bought Listerine did so because they relied on the allegedly false claim regarding dental floss. Customers who bought the Listerine for other reasons—maybe they liked the taste or wanted fresh breath—can’t be in the class, the court decided.

For plaintiffs to now get class action status against an advertiser, “there must in fact have been reliance upon the alleged misstatement” in the ad, said one of the lawyers, "not a mere likelihood of reliance."

The case may still be potentially appealed to the California Supreme Court.

“If this decision stands, it will constitute the biggest rollback of consumer protections ever in California history,” said a lawyer working on the case.

The lawsuit seeks to represent any person who purchased Listerine on the basis that it was as effective as dental floss.