With EU regulatory approval in the bag and US regulatory scrutiny expected to clear in the coming weeks, the €55bn AbbVie and Allergan merger should start in earnest in February or March.

AbbVie and Shire agreed to terminate what would have been the biggest US tax inversion after AbbVie pulled its support for the deal in the wake of proposed changes to US rules governing such transactions.

SHIRE chief executive Angus Russell said he understands why Sanofi-Aventis SA may buy Genzyme, his biggest rival in producing drugs for rare genetic diseases.