Results from a new study have suggested that Sanofi-Aventis’ insulin drug Lantus could raise the risk of cancer although some medical experts are saying the studies are inconclusive.

Four new studies of 300,000 patients released on Friday in the journal Diabetologia, the journal of the

European Association for the Study of Diabetes, raised concerns over possible links between the popular diabetes drug Lantus and cancer.

Widely used since 2000 by millions of people with type 1 and type 2 diabetes, Lantus which is an artificial form of insulin known as insulin analogue had sales of $3.43 billion last year. The drug, known generically as glargine insulin is popular as it needs to be injected once a day as compared to the older insulins which are twice or more a day.

Human insulin is a hormone normally produced by the pancreas that helps the body use glucose for energy, but Lantus has a slightly altered molecular structure that allows it to last longer in the body.

Dr. R. Paul Robertson, president of the American Diabetes Association and a diabetes researcher at the University of Washington in Seattle said, "It's frequently prescribed in the United States. When I have patients with type 1 or type 2 diabetes, I give that drug. It's been very effective. "

The studies were conducted by researchers in four European countries who reviewed data of patients taking Lantus and other insulins. One study involving 127,000 Germans found the risk of being diagnosed with cancer over an average of 1½ years was 9% higher in patients taking a low dose of Lantus than in people taking human insulin, which is an older kind of insulin. The cancer risk was 31% higher in patients taking a higher dose of Lantus.

A  Swedish study found that patients taking Lantus had double the risk of breast cancer compared with patients taking other insulins though the number of breast cancer cases was small, the European diabetes association said. A study in the U.K. showed no link between Lantus and cancer.

The European association said patients taking Lantus in the studies were generally older, more overweight and had higher blood pressure, making it possible that "any difference in cancer risk could be attributed to the pre-treatment characteristics of the groups."

Sanofi Chief Executive Christopher Viehbacher said Sanofi plans to investigate the matter. "I don't think the medical community is nearly as impressed with these results," he said calling the new studies inconclusive and flawed, adding that clinical trials in Sanofi's database involving 70,000 people have shown no link between Lantus and cancer.

Dr. Louis Philipson, of the University of Chicago Medical Center said it may be that patients taking Lantus are simply exposed to insulin longer, and if they have any underlying tumors, the drug may accelerate tumor growth. "Even though this information is early and the results are conflicting, it's still the case that using less of this drug is a good idea," he said.

The American Diabetes Association called the studies "conflicting and inconclusive," and cautioned "against over-reaction until more information is available." They also recommended that patients continue on the treatment, and consult their doctor before making any changes.

The European Medicines Agency called the study results "inconsistent" and said a relationship between Lantus and cancer "cannot be confirmed nor excluded" as a result. It advised patients taking Lantus to continue doing so, and called for further study of the matter.