UPDATE 3-US FDA plays down cancer link with Sanofi's Lantus

* FDA says studies may be too short to assess risk

* Questions whether cancer risk "truly exists"

* Sanofi shares up 3.1 percent (Adds doctor comment in graphs 8-9)

By Lewis Krauskopf and Susan Heavey

NEW YORK/WASHINGTON, July 1 (Reuters) - The U.S. Food and Drug Administration on Wednesday cast doubt on recent studies that suggested possible cancer risk with Sanofi-Aventis' widely used Lantus diabetes drug, boosting investor confidence in the blockbuster product.

Shares of the French drugmaker, which have stumbled in the past week over Lantus concerns, rose 3.1 percent in Paris after the FDA also said patients should not stop taking their therapy without consulting a physician.

In a communication posted on its website, the agency questioned four recently published studies over the cancer link, saying they did not track patients long enough to properly evaluate any such risk from the drug.

Additionally, the FDA said, "inconsistencies in findings within and across individual studies raise concerns as to whether an association between the use of (Lantus) and cancer truly exists."

It added that there may be differences in the patients studied that could have contributed to the studies' findings.

Shares of the French drugmaker have plunged about 13 percent in the last week amid concerns that patients and doctors worried about Lantus' safety would stop using the product and turn to rival treatments such as Novo Nordisk's long-acting insulin Levemir.

Lantus is a top seller for Sanofi, and it is seen as particularly important as other key Sanofi products -- blood thinners Plavix and Lovenox -- face possible generic competition. Lantus tallied 2.45 billion euros ($3.5 billion) in 2008 sales, a figure analysts have expected to grow over the next five years.

Dr. Richard Bergenstal, president-elect of medicine and science with the American Diabetes Association, said it appears the FDA was taking the risk posed by the Lantus studies "seriously but cautiously."

"I do think they were saying when you get confusing data or conflicting data, don't make clinical decisions until we get more," Bergenstal said.

Sanofi has stood behind the safety of its diabetes drug, and a company spokesman in Paris said on Wednesday it was "committed to working with the FDA and other health authorities and scientific experts to clarify the situation."

The FDA said it would continue reviewing the safety data for Lantus, including the new studies, to better understand any possible cancer risk.

Earlier this week, the European Medicines Agency said it was asking its scientific experts to perform a detailed assessment of the European studies' results and any other relevant information, since a link between Lantus and cancer "cannot be confirmed nor excluded."

Mark Dainty, an analyst at Citigroup, said the stock's recent drop meant the market was already discounting a worst-case scenario.

But the actual decline in Lantus sales might not be as severe as the 60 percent drop seen in global sales of GlaxoSmithKline's Avandia, which was linked to heart attack risk in 2007, he said.

(Reporting by Lewis Krauskopf and Susan Heavey; Additional reporting by Ben Hirschler in London; Editing by Lisa Von Ahn, Derek Caney and Richard Chang)