As the battle for US obesity market share hots up, Eisai plans to have 400 sales reps promoting Belviq, the weight loss treatment it has licensed from Arena Pharmaceuticals, by the end of the year.

This means that the field force supporting Belviq (lorcaserin) will have doubled since its launch in June 2013. The expansion has already begun and, with increases in coverage of the drug by health plans and pharmacy benefit managers, Eisai says it will be able to to reach 65,000 physicians in the United States, including primary care providers, endocrinologists, cardiovascular and gastrointestinal specialists.

Lonnel Coats, chief executive of Eisai's operations in the USA, said the sales force increase "demonstrates the success we have gained in discussing the potential value of Belviq in chronic weight management within the payor community". He added that the firm "fully expects the reimbursement landscape will further improve as we head into 2014".

Belviq is competing with Vivus' Qsymia (phentermine/topiramate) and to date, the launches of both drugs have been disappointing.

UPDATE: In an investor note, Cowen & Co analyst Simos Simeonidis wrote that the move demonstrates Eisai’s further commitment to supporting the drug, adding that it will provide an incremental commercial advantage to Belviq that Vivus will have to deal with, and makes Vivus' need for a pharma partnership even greater". However, he added that "this change in the commercial marketplace does not change the drug's clinical profile, and this is where we believe Belviq is at a disadvantage".