(Reuters) - The U.S. Food and Drug Administration said on Monday it had approved Allergan Plc's drug for relieving migraine headaches after their onset.

The drug, Ubrelvy, is the first oral version of a new class of drugs called calcitonin gene-related peptide (CGRP) inhibitors to win the FDA's approval for treating the neurological disease. (bit.ly/2QeY1b7)

This new class of drugs act by interfering with CGRP, a protein involved in causing severe headaches.

Allergan, which has been pushing to solidify its presence in the migraine market, is developing another oral CGRP inhibitor for migraine prevention, called atogepant.

With the new drugs, the company hopes to expand beyond selling Botox, a purified form of botulinum toxin that also prevents chronic migraine.

Offering drugs for both prevention and treatment of migraine, Allergan will have "the most complete" product line for neurologists and primary care physicians who are managing the condition, Chief Commercial Officer Bill Meury told Reuters.

"We say internally that we are building 'Migraine Incorporated' inside of Allergan."

Rival products such as Amgen Inc and Novartis AG's Aimovig, Eli Lilly and Co's Emgality and Teva Pharmaceutical Industries Ltd's Ajovy won approval within months of each other last year, signaling a threat to Botox's position as a therapeutic.

Click here for an interactive on sales estimates for migraine drugs: (tmsnrt.rs/2ZhIzPS)

Botox, which is used mainly as a cosmetic treatment to alleviate wrinkles, has also been facing competition from new entrants to the medical aesthetics space, such as Evolus Inc's Jeuveau.

Ubrelvy's approval helps Allergan beat rival Biohaven Pharmaceutical Holding Company Ltd by winning an FDA nod for the first oral CGRP inhibitor for treating acute migraine.

Allergan is focused on positioning Ubrelvy as an alternative for patients who are taking triptans, a mainstay in the treatment of acute migraine.

While triptans have made a big difference in patients' lives, 30% of patients do not respond sufficiently to them and 20% have cardiovascular risk factors that makes them unsuitable for using the drug, Meury said.

GlobalData's Maura Musciacco forecasts Ubrelvy's sales to reach $486 million in 2025.

(Reporting by Manojna Maddipatla in Bengaluru; Editing by Shinjini Ganguli)