Harrison will join the pharma firm later this year as a director in its corporate media relations team. Cadbury is looking for a financial comms chief to replace her.
A source at Cadbury said: 'Alex has a long-held desire to work in the pharma sector, having trained as a scientist, and so our loss is GSK's gain.'
Cadbury also recently lost its global corporate comms director Katie Bell after a restructure of its corporate affairs team (PRWeek, 4 February).
But it has now hired Dionne Parker as global head of corporate comms. She previously held roles at Hogg Robinson and Whitbread.