We finally got a big strategic deal in the mergers and acquisitions market, but it’s one that may never come to fruition.

Express Script’s deal to acquire Medco Health Solutions for roughly $29 billion in cash and stock is being applauded by ESRX shareholders, but the key constituency on whether this deal will happen is likely to be the Federal Trade Commission.

The risk that the deal will be challenged by the FTC is why shares of Medco, while up nicely, are trading a full 10 points below the implied price of the transaction.

One key here is the definition of so-called “national players” in the pharmacy benefits manager (PBM) sector. At present there appear to be three national players—CVS Caremark, Medco, Express Scripts—but with UnitedHealth Group taking back its PBM business from Medco at the end of next year, it may be considered a true competitor for national PBM contracts.

Four national players going to three may get by the FTC, but three going to two is likely a no-go.

The companies are already making plenty of noise about how the combined company will be able to contain health care costs in an environment where doing so is priority number one.

But that doesn’t mean this deal won’t become a political football. One only need go back a few years when Express tried to buy Caremark to recall that 22 state attorneys general came out against the proposed transaction.

Follow David on Twitter:

Questions?  Comments?  Write to faberreport@cnbc.com.