Over the weekend, Celgene (NASDAQ:CELG) bought a drug and a half. While that purchase won't have huge potential for increasing sales in and of itself, the synergies that newly acquired Pharmion (NASDAQ:PHRM) provides should help Celgene justify the $2.9 billion price tag.

Pharmion sells Vidaza, which is approved in the U.S. for treating high-risk myelodysplastic syndromes (MDS) patients. A problem in these patients' bone marrow leads to underproduction of red or white blood cells. Pharmion will seek European approval for Vidaza by the end of the year, and it also owns rights to sell Celgene's multiple myeloma drug, Thalomid in Europe. (That'd be the half-a-drug.)

Combine that drug-and-a-half with Celgene's own Revlimid, which is approved for treating lower-risk MDS and multiple myeloma, and another multiple myeloma drug, Alkeran, which Celgene licensed from GlaxoSmithKline (NYSE:GSK), and the new Celgene has a four-drug combo (well, three and a half, technically) that could be sold by a single sales force.

The combined sales force -- especially in Europe, where Pharmion is more built-up -- should help Celegene compete against the likes of Johnson & Johnson (NYSE:JNJ). J&J holds the European rights to a competing multiple myeloma drug, Velcade, from Millennium Pharmaceuticals (NASDAQ:MLNM).

The $2.9 billion question is whether Celgene is overpaying. That price is 11 times Pharmion's trailing 12-month revenue of $256 million. That's roughly the same ratio that AstraZeneca (NYSE:AZN) spent to take over MedImmune earlier this year. However, given the sales-force synergies, and Pharmion's two drugs in mid-to-late clinical trials, Celgene should be able to justify the purchase price in the not-too-distant future.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Johnson & Johnson and Glaxo are selections of the Income Investor newsletter. The Fool's disclosure policy is immune to takeovers.