The review, which is being handled by ID Comms, comes less than three years after GSK awarded its European business to MediaCom, while PHD was handed its US media account.
Both agencies had retained the accounts in 2015 following a global media review, after having been first appointed in 2013.
A year later, GSK completed an agency review for the other parts of its marketing comms business by hiring nine agencies, including Grey and Saatchi & Saatchi for advertising.
GSK's agencies were appointed to work on the company's seven power brands (Sensodyne, Parodontax, Poligrip, Voltaren, Panadol, Theraflu and Otrivin) and 12 core brands (Biotene, Contac, Flonase, Excedrin, Fenbid, Tums, ENO, Horlicks, Physiogel, Lamisil, Fenistil and Abreva/Zovirax).
GSK did not respond to a request for comment by Campaign UK by press time while MediaCom and PHD would not comment and referred Campaign to GSK.