One would think that getting a combination pill of two already approved medications would be rather simple, but Merck (NYSE: MRK) seems to have messed it up.

Again.

The pharma giant said today that the Food and Drug Administration rejected its marketing application for a combination of cholesterol drugs Zetia and Pfizer's (NYSE: PFE) Lipitor, which is now available as a generic.

When Merck originally submitted the marketing application back in 2009, the FDA refused to even accept it, sending it back stamped with a schoolteacher-like INCOMPLETE.

Merck fixed the missing manufacturing and stability data, but now the agency reportedly wants more clinical data about the drugs being taken in combination. Maybe the FDA is just being ridiculously picky, but it seems weird that Merck can't figure out what the agency needs to approve the combination pill. It's not like this is the first combination pill Merck has produced. It's not even the first one with Zetia, which is combined with Zocor to form Vytorin.

Zocor and Lipitor are both statins, which work in a different way from Zetia to lower cholesterol. But Lipitor is more powerful than Zocor, so having a combination product would be incrementally beneficial, as patients who aren't controlling their cholesterol with Lipitor could add the active ingredient in Zetia without having to take another pill. And that's become more important with the introduction of generics, since many new patients will start on Lipitor and some might even switch from Zocor or AstraZeneca's (NYSE: AZN) statin, Crestor.

Ironically, even with all the delays getting the combo product approved, the delay might not cost Merck anything. While there are generic versions of Lipitor on the market, they're there because of deals Pfizer made with Ranbaxy Laboratories (OTC: RBXZF) and Watson Pharmaceuticals (NYSE: WPI). Pfizer sued Merck, claiming that its patents are still valid. If the two sides can't come to an agreement and Merck can't win in court, approval could be held up until early 2014.

Hopefully, by then Merck will have figured out what the FDA wants.

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