NEW YORK (Reuters) - Shares of Pozen Inc (POZN.O: Quote, Profile, Research) soared more than 40 percent on Wednesday after U.S. health regulators finally approved the combination migraine treatment it developed with GlaxoSmithKline Plc (GSK.L: Quote, Profile, Research).

The drug, Treximet, which combines the common pain drug naproxen with Glaxo's older, widely used migraine treatment Imitrex, is expected to be available in the United States by mid-May, the companies said Tuesday night.

The approval by the U.S. Food and Drug Administration triggers a $20 million milestone payment to Pozen, which will also receive royalties from Glaxo on sales.

Lazard Capital Markets forecast Treximet sales of $58 million this year, with annual sales reaching a peak of $320 million.

Jefferies & Co analyst Eun Yang said the approval news was far better than his "anticipation of a further delay in final approval."

Yang forecast annual peak Treximet sales of $200 million and peak annual royalty revenue of $34 million for Pozen.

Analyst enthusiasm was somewhat tempered by the potential impact of competition from generic Imitrex later this year, as well as concerns over how successfully the companies will be able to switch patients over from Imitrex.

But they said the Treximet approval could signal an improved regulatory profile for Pozen's next pain drug, which is in late stages of clinical testing.

Treximet's U.S. approval met with several delays despite its being a combination of two older approved medicines.