It's easy to look at the headline for clinical trials and think everything is fine. After all, the drug met its primary endpoint and that's a good thing, right? Sure, but there's often more to the story.

This week's "That's great but ..." moment comes from sanofi-aventis (NYSE: SNY). Yesterday, the company released top-line data for its phase 3 diabetes drug candidate, lixisenatide. The trial was a success, but I wouldn't pencil it in as a blockbuster just yet.

In the trial released yesterday, Sanofi compares the drug to placebo. That's an OK strategy for diseases where there are no other treatment options, but there are numerous options for pre-insulin diabetics, including Merck's (NYSE: MRK) Januvia and GlaxoSmithKline's (NYSE: GSK) Avandia. Sanofi is testing the drug in head-to-head trials against Amylin Pharmaceuticals (Nasdaq: AMLN) and Eli Lilly's (NYSE: LLY) Byetta and in combination with metformin, Takeda's Actos, and its own insulin drug Lantus.

Until that data is available, we'll have to withhold judgment about how well the drug works. It looks promising that 46.5% to 52.2% of patients on drug reached the target HbA1c level -- a measure of long-term glucose levels -- compared with 26.8% of those taking placebo, but comparing the data to other trials is an exercise in futility. The patient populations are likely to be different, and the procedures can vary, with some drugmakers requiring a wash-out of other drugs while others don't.

Sanofi's bigger problem is that it trails Byetta and Novo Nordisk's (NYSE: NVO) Victoza, which are in the same class and already on the market. Lixisenatide is injected once a day, the same as Victoza and better than Byetta's twice-daily dosing. But Amylin and Eli Lilly's follow-up, Bydureon, is only injected once a week thanks to Alkermes (Nasdaq: ALKS) extended-release technology. Even with comparative data, it'll be difficult to gain much traction if lixisenatide hits the market a year or two behind Bydureon.

A successful clinical trial makes for a good press release, but it's going to take a lot of luck and perhaps a few stumbles by its competitors for lixisenatide to be successful.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool owns shares of Glaxo and has a disclosure policy.