For a pharmaceutical company, the only thing better than having a drug with more than $1 billion in annual sales is getting it approved to treat more patients.

Eli Lilly (NYSE:LLY) scored a second indication for its osteoporosis drug EVISTA when the FDA approved it to reduce the risk of invasive breast cancer in postmenopausal women with osteoporosis or women who are at high risk for invasive breast cancer.

EVISTA is a selective estrogen receptor modulator (SERM), which acts differently on estrogen receptors in different parts of the body. EVISTA appears to prevent breast cancer by blocking estrogen from stimulating the growth of tumor cells.

The new indication is coming at the perfect time. Merck's (NYSE:MRK) competing osteoporosis drug, Fosamax, is going off-patent in February, so the fact that it prevents invasive breast cancer might keep some EVISTA patients from switching to cheaper generic versions of Fosamax that are sure to flood the market.

Being able to market the drug for the prevention of breast cancer should also help EVISTA compete against Roche's and GlaxoSmithKline's (NYSE:GSK) BONIVA, as well as Wyeth's (NYSE:WYE) bazedoxifene, which Wyeth is hoping to seek approval on.

While the approval will certainly help EVISTA compete against other osteoporosis drugs, it's not clear how many patients will take the drug just for the prevention aspect. The new approval doesn't increase the patient population by that much -- there are many more women with osteoporosis than those at risk for invasive breast cancer. Given that the drug will still contain a "black box" warning regarding patients with a past history of venous thromboembolism or a risk of stroke, doctors may be a little leery of prescribing it strictly for its preventative aspects.

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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.