Bristol-Myers Squibb (NYSE: BMY) and AstraZeneca (NYSE: AZN) announced yesterday that the Food and Drug Administration had changed the label of their diabetes drug Onglyza to include data on how the drug works in renal-impaired patients.

How much is it going to help the companies in their chase to catch Merck's (NYSE: MRK) Januvia? Not much is my guess.

Onglyza isn't even in the same league as the entrenched diabetes drug leader.

Drug

Sales in Fourth Quarter 2010 (in millions)

Onglyza containing drugs

$73

Januvia containing drugs

$962

Source: Company releases.

Many diabetics have renal impairment, which can affect the efficacy of a drug. Having that information on the label is important. But Januvia's label already contains information about patients with renal impairment, so Onglyza isn't offering anything new.

With the additions only putting Onglyza on equal footing, there's still no compelling reason for doctors to switch patients from Januvia to Onglyza. And doctors' experiences with Januvia means new patients are likely to be prescribed Januvia as well. In the fourth quarter, sales increased 21% year over year.

As far as I can tell, the only advantage that Onglyza has over Januvia is when they are combined with a generic metformin into a single pill. Janumet -- Januvia plus metformin -- has to be taken twice a day, but Kombiglyze -- Onglyza plus metformin -- only has to be taken once a day.

But Bristol-Myers and AstraZeneca won't have much of an advantage for long. Merck has filed for approval of a once-daily pill using Depomed's (Nasdaq: DEPO) extended-release technology.

That's the problem with developing me-too drugs. It's hard to gain any traction.

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