Britain OKs Allergan's Botox drug for migraine

* First country to approve Botox as migraine treatment

* Approval in other European countries expected (Adds analysts' comments)

LONDON, July 9 (Reuters) - Britain has become the first country to approve Allergan Inc's (AGN.N) anti-wrinkle injection Botox as a treatment for migraine, opening up a new market for the product.

U.S. drugmaker Allergan said Botox was licensed by British regulators on Friday for the prevention of headaches in adults with chronic migraine.

Historically, treatment for chronic migraine has consisted of a combination of over-the-counter and prescription treatments such as triptans, tricyclic antidepressants, beta-blockers and anti-epileptics.

"The authorisation of Botox marks an evolution in medical care for the prevention of headache in adults with chronic migraine. It is also a significant milestone in the history of Botox and Allergan," said Scott Whitcup, Allergan's head of research and development.

Ronny Gal, an analyst at Sanford Bernstein, said he expects approval in other European countries will follow but that U.S. approval is not guaranteed.

Wells Fargo analyst Larry Biegelsen estimates that Botox sales for migraine in the European market could reach $200 million to $300 million.

"We believe the commercial opportunity for Botox in Europe is smaller than the U.S. but still meaningful longer term," Biegelsen said in a research note earlier this week. (Reporting by Ben Hirschler; additional reporting by Lewis Krauskopf in New York; Editing by David Holmes and John Wallace)