Shares in sanofi-aventis fell as much

as 7.2 percent on Friday, after a study published in the journal Diabetes Care suggested that higher doses of the company's diabetes drug Lantus (insulin glargine) may be linked to an increased risk of cancer.

The French drugmaker's chief medical officer Jean-Pierre Lehner called the trial "unclear," adding that “you can’t reach reliable conclusions with a study on 1500 patients.”

In the study, Italian researchers focused on 112 people with diabetes who developed cancer, and compared them to 370 control subjects. During the six-year review, the number of people who received each type of insulin was similar, but the data show that a link to an increased risk of cancer was only observed in those patients who received higher doses of Lantus, the researchers note.

Mirabaud Securities analyst Nick Turner remarked that "there’s no difference in what this study is showing and what was published last year” in the journal Diabetologia, when Lantus was first linked to an increased risk of cancer. “Clearly the risk is relatively light. If anything, this study shows that the risk could be mitigated with dosage, which is a positive,” he added.

However, Bernstein analysts noted that the study in Diabetes Care "has several advantages in terms of design compared to other studies we have seen so far. There was long follow up and this is very important because cancer takes a long time to develop." The analysts added: "Most of the work that has shown Lantus as 'innocent' has had very short follow up; too short to plausibly show a cancer risk in some cases. The investigators also appear to have had good data on things like co-morbidities, insulin dosing, blood glucose, etc.