Aug 14 (Reuters) - The U.S. health regulator has approved expanding the use of Revance Therapeutics' Daxxify to treat a painful neck muscle condition, intensifying the anti-wrinkle injection's rivalry with AbbVie's Botox.

The approval by the Food and Drug Administration helps Revance enter the $2.5 billion U.S. market for therapeutic neuromodulator, a method that directly acts on nerves, the company said on Monday.

Cervical dystonia accounts for about $345 million of that market. The painful condition involves involuntary contraction of neck muscles and causes the head to twist to one side.

Revance will move into a preview phase with some doctors and key opinion leaders (KOLs) initially after the approval, company executives told Reuters before the decision.

"We'll use that group as our faculty to then train that broader launch group which will be in 2024," President Dustin Sjuts said.

Revance said it would share more information on the pricing when the product is launched commercially.

The company has been trying to attract clients to Daxxify from its more popular rival with its "Break Up with Botox" marketing campaign.

The newer anti-wrinkle injection recorded $22.6 million in sales in the second quarter.

The California-based company is counting on Daxxify's longer treatment duration to attract more doctors and patients from Botox.

Most cervical dystonia patients saw the treatment effect last about 20 weeks and 24 weeks when using different dosages of Daxxify, a late-stage study showed, compared with three months for Botox, as per FDA data.

This was similar for Daxxify when it had secured U.S. approval as an anti-wrinkle injection last year. (Reporting by Leroy Leo in Bengaluru; Editing by Sriraj Kalluvila)