It's no easy feat to get a COX-2 inhibitor approved in the U.S. these days. Just ask Merck (NYSE:MRK), still suffering from its recall of Vioxx.

COX-2 inhibitors are potent compounds used to treat individuals suffering from osteoarthritis and other pain conditions requiring anti-inflammatory drugs. In its heyday, Merck was selling $2.5 billion worth of Vioxx. But allegations that the drug increased users' risk of cardiovascular disease torpedoed Vioxx's fortunes in 2004, and Merck's still wading through the resulting pile of product liability lawsuits. With sales of approximately $2 billion in 2006, Pfizer's (NYSE:PFE) Celebrex now virtually owns the market for COX-2 inhibitors.

In April, the FDA rejected Merck's attempt to gain approval for another COX-2 inhibitor, called Arcoxia. The FDA followed the overwhelming sentiment of a panel of federal health advisors, which voted 20 to 1 against the drug's approval. The vote effectively protected Pfizer's command of the U.S. COX-2 inhibitor market. But if Pfizer's celebrating that decision, its jubilation may be short-lived.

On Friday, Swiss pharmaceutical giant Novartis (NYSE:NVS) reported that data from a new study of 741 patients revealed that those treated with its COX-2 inhibitor Prexige experienced a significantly smaller effect on blood pressure, compared to ibuprofen. Prexige is under review by the FDA, and it's already approved in 55 countries.

In retrospect, it's not surprising that the U.S. hasn't yet approved Arcoxia. Just prior to the safety panel's vote, an FDA safety expert testified that the drug may substantially increase patients' risk of stroke and heart attack. I'm sure that this notion weighed heavily on panel members' minds, given the similar concerns that surrounded Vioxx's recall.

It appears that this study by Novartis on Prexige aims to specifically address such concerns, and the latest results are favorable. The FDA may still hesitate to approve another COX-2 inhibitor so after the Vioxx debacle, but Novartis appears to have presented a convincing case for Prexige's relative safety. The potential sales any Celebrex competitor might generate in the U.S., provided it's approved, might make Novartis shares worth a second look for patient Fools.

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Fool contributor Billy Fisher does not own shares of any of the companies mentioned. Pfizer is an Inside Value recommendation. The Fool has a disclosure policy.