It seemed like a good idea. Both Abbott Labs' (NYSE: ABT) TriLipix and AstraZeneca's (NYSE: AZN) Crestor are already on the market. They're used together to improve patients' cholesterol and lipid levels. Why not combine them into a single pill, which would make them easier for patients to take, and might attract a few patients away from competing cholesterol drugs? Sounds like a win-win, to me. The FDA apparently thought otherwise.

Combo pills are fairly common for heart drugs. Merck (NYSE: MRK) merged its Zocor and Zetia to form Vytorin, and it plans to combine Zetia with Pfizer's (NYSE: PFE) Lipitor once the latter goes off-patent. They're also common in diabetes drugs, where Merck's Januvia, GlaxoSmithKline's (NYSE: GSK) Avandia ,and Takeda's Actos are all available as combo product with generic versions of Bristol-Myers Squibb's (NYSE: BMY) Glucophage.

In that light, approval for Abbott and AstraZeneca's drug, called Certriad, probably seemed like an afterthought -- until today, when the duo received a complete response letter from the Food and Drug Administration.

In typical pharma fashion, the companies didn't give any indication what problem the FDA had with Certriad. The agency's "request for additional information" (that's all the companies revealed) could be anything from a simple lab test looking at the interaction of the drugs, to a new trial measuring clinical outcomes like heart attacks in patients taking the combo drug.

The latter seems harsh, because both drugs are already on the market. But considering the lack of clinical effectiveness that TriLipix's older brother TriCor showed earlier this month, it's not beyond the realm of possibility. A request for an outcomes trial would kill any chance of Certriad reaching the market soon (if at all), because it takes many years to gather heart-related data.

Until the companies share additional information, it's a little hard to know what the future holds for Certriad. At least investors can find solace in knowing that TriLipix and Crestor are still available individually.

Austin Edwards wonders whether it's too late to cash in.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool owns shares of GlaxoSmithKline. Combine the Fool's disclosure policy with a nice glass of wine for added enjoyment.