LONDON - Sanofi-Aventis's diabetes drug Lantus may increase the risk of cancer, according to new but inconclusive studies from experts around Europe.

Shares in the French drugmaker have tumbled on fears the news will undermine sales of a medicine Sanofi is relying on to sustain growth at a time when other drugs face generic competition.

Here are some questions and answers on the issue:

WHAT DOES THE EVIDENCE SHOW?

Four retrospective studies of registries covering 301,136 insulin-treated patients in Germany, Sweden and Britain suggest Lantus may increase the risk of cancer, though the data is not conclusive. The smallest study found no signs of a link.

The association was noted particularly for breast cancer, but the absolute risk was small. The largest German study found a difference of roughly one extra cancer diagnosed for every 100 people taking Lantus compared with those on human insulin.

Sanofi points out that previous clinical studies, which represent the gold standard of evidence, indicate no association between Lantus and cancer.

WHY MIGHT LANTUS BE LINKED TO CANCER?

Insulin treatments can bind to two different receptors, or molecular doorways, on the surface of cells -- insulin and insulin-like growth factor 1 (IGF-1). The first mainly causes glucose lowering whereas the latter mainly induces cell proliferation.

The fact that some artificial versions of insulin, called analogues, have a structure making them more likely to bind to the IGF-1 receptor could explain a cancer link.

The theoretical risk of cancer from insulin analogues has been raised in the past. U.S. Food and Drug Administration review documents dating back to 2000 mentioned findings of malignancies in rodents. An earlier clinical trial with Lantus also noted an increase in progression of retinopathy, an eye disease that is characterised by cell proliferation. The European Association for the Study of Diabetes (EASD), says there is no evidence Lantus actually causes cancer; at worst it might promote growth of cancers that are already present but have yet to be diagnosed.

HOW WILL DOCTORS REACT?

Diabetes experts recommend patients should not stop treatment. They could, however, consider using alternative therapies. The American Diabetes Association warned against over-reaction to a set of research papers that it said were conflicting and inconclusive.

However, the experience of past drug scares suggests patients and many doctors will be wary, especially since alternative treatment options are available from other drug companies.

Medical experts will discuss the matter in more depth at a special symposium during the annual meeting of the EASD in Vienna in September.

WHY IS LANTUS SO IMPORTANT TO SANOFI?

More than a fifth of Sanofi's current drug sales face patent loss and generic competition by 2012. But Lantus had been seen as a big and secure sales driver for the next five years or so.

Lantus sales grew 28 percent to 2.45 billion euros ($3.43 billion) last year and analysts had been looking for that to reach 4-5 billion by 2012. Such bullish forecasts now look out of reach. (Reporting by Ben Hirschler; Editing by Greg Mahlich)

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