The FDA approved over-the-counter use of Sanofi-Aventis allergy drug Allegra, paving the way for launch by the French drug maker’s US consumer healthcare unit, Chattem.

The company said it plans to have OTC Allegra on pharmacy shelves by March. A Sanofi spokesperson declined to provide specifics on marketing plans.

Sanofi bought Chattem last year for $1.9 billion, and at the time CEO Chris Viehbacher hailed the acquisition for giving the firm a US marketing platform for future Rx-to-OTC switch brands, starting with Allegra.

Hanspeter Spek, Sanofi president, global operations, echoed that sentiment yesterday, stating that converting prescription medicines to OTC products is “a key growth driver for Sanofi-Aventis to become a diversified healthcare company also in the United States.” The firm’s US sales have taken a hit from recent approvals of generic versions of blood thinner Lovenox and cancer drug Taxotere.

The Allegra approval was expected, and Sanofi’s increased exposure into the consumer segment, via the Chattem acquisition, will enable it to leverage the brand, Jeffries International analyst Jeff Holford, who covers Sanofi from London, told MM&M. However, beyond Allegra, Rx-to-OTC switches may not to be an immediate growth driver for the company.

“As most of Sanofi’s remaining branded blockbuster drugs are for serious indications (cancer, cardiovascular disease) they do not have many more such large opportunities for OTC switches as this in the near to mid-term,” Holford wrote in an e-mail.

Plavix, a blockbuster antiplatelet drug which Sanofi co-markets with Bristol-Myers Squibb, will face generic competition in May 2012. (The companies announced yesterday that pediatric exclusivity extends their right to exclusively market Plavix another six months.) Cancer med Eloxatin, another big seller, goes off patent in August 2012.

Allegra has been on the market since 1996, and sales have decreased the last year as generic competition has arrived. The FDA approval covers the Allegra family of allergy products and is for adults and children 2 years and older. The OTC versions will be available in original prescription strengths, the company said.