An ounce of prevention is worth a pound of cure. It's also worth big bucks.

Unfortunately, it doesn't look like Merck (NYSE: MRK) and GlaxoSmithKline (NYSE: GSK) will be able to partake.

Merck and Glaxo are trying to get their prostate drugs -- Proscar and Avodart respectively -- approved to prevent prostate cancer. Both are currently approved to shrink enlarged prostates, but preventing cancer would open up a whole new area of potential sales.

But both got shot down by a Food and Drug Administration advisory panel yesterday. Proscar was rejected by a unanimous vote. Avodart fared better, but only by a little: The vote was 14-2 that the drug's benefits didn't outweigh the risks.

In clinical trials, the drugs decreased incidence of low-grade prostate cancer tumors by nearly 25%. But the men in the study got high-grade tumors at a higher rate. The benefit of preventing less-severe tumors that are often not fatal didn't outweigh the possibility that the drugs were causing a small number of high-grade tumors that are often deadly.

Ironically, considering the data, advanced prostate cancer drugs such as sanofi-aventis' (NYSE: SNY) Taxotere and Dendreon's (Nasdaq: DNDN) Provenge might actually benefit if the FDA approves Merck's and Glaxo's preventative drugs, if they in fact lead to an increase in the number of advanced prostate cancer cases.

The low-grade tumors are usually treated with radiation or surgery, so not having a drug on the market that prevents them will mostly benefit doctors that perform the procedures, as well as companies such as General Electric (NYSE: GE) and Intuitive Surgical (Nasdaq: ISRG) that make radiation and surgical equipment.

The FDA has the final say, but considering the opinion of the panel of outside experts, I doubt the agency will approve the drugs. Merck and Glaxo will have to be satisfied with their current roles treating enlarged prostates.

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Intuitive Surgical is a Motley Fool Rule Breakers pick. The Fool owns shares of and has written covered calls on GlaxoSmithKline, which is a Motley Fool Global Gains choice. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

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