It's not exactly a new indication, but a label change the FDA granted Schering-Plough (NYSE: SGP) will allow it to sell more of a drug to some patients. Unfortunately, it also approved a change to use less of the drug in other patients.

Yesterday, the FDA approved changes to Schering's Rebetol, which is taken in combination with its Pegintron to fight hepatitis C infections. The new label indicates that patients will have a higher cure rate if doctors prescribe Rebetol based on the patient's weight -- 800-1,400 mg daily -- compared with the 800 mg dose that was previously suggested.

The same clinical trial that showed higher doses of Rebetol helped larger patients, also showed that a six-month treatment, rather than the standard one year, was sufficient to wipe out the virus in patients that have genotype 2 or 3 infections. Those patients will be taking less of the drug than before, but fortunately for Schering, genotype 1 is the most common type in North America.

Without knowing the size of the patients and the exact breakdown of genotypes that have infected patients taking the Pegintron/Rebetol combination, it's difficult to know how the new label will affect the drug combo's sales. My gut feeling is that even if it doesn't increase sales, it should help in the long run, since the increased cure rate from the higher doses and easier dosing regimen for genotype 2 and 3 patients should help it fight off upcoming competition.

With so many promising hepatitis C treatments in the works, Schering and Roche need to do whatever they can to keep their cash-cow Pegylated introns -- Pegintron and Pegasys -- as the lead drugs for as long as possible. They are both developing different antiviral compounds, but smaller companies such as Vertex Pharmaceuticals' (Nasdaq: VRTX) and Pharmasset (Nasdaq: VRUS) are right on their heels.

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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.