US drug regulators have ordered Aventis Pharmaceuticals to halt a "false or misleading" ad campaign.

The Food and Drug Administration contacted the pharma giant after investigating TV commercials, print ads and a fulfilment letter for antihistamine Allegra, the firm's biggest-selling drug.

Extracts from the FDA's letter to Aventis have now appeared on AdAge.com, revealing that the regulator was unhappy with the wording of some of these ads.

The advertising copy allegedly described Allegra as "the number one prescription antihistamine", adding that it is "specifically designed to block the histamine that triggers allergic responses," and "clinically superior to other prescription antihistamines."

In its letter, however, the FDA argues that similar prescription allergy treatments such as Zyrtec and Clarinex "have the same [anti-allergy] mechanism of action … your suggestion of a unique mechanism and superior effectiveness for Allegra is false or misleading."

The regulator ordered French-headquartered Aventis to halt distribution of the offending material and draw up a plan for responding to the charges.

The ads were created by McCann-Erickson Worldwide in New York. Allegra receives a lot of ad support: over the first ten months of last year, Aventis spent $98.5 million (€78.0m; £53.9m) on the brand according to TNS Media Intelligence/CMR.

Data sourced from: AdAge.com; additional content by WARC staff