Approval from the board at Creston was passed at the firm's AGM on 15 December. The total cash consideration of up to £19.5 million comprises a £5.9m initial consideration payable on closing and an additional earn-out of up to £13.5m payable in two instalments in 2013 and 2015.
Cooney/Waters and Alembic Health Communications, together the Cooney/Waters Businesses, were founded in 1992 and employ around 50 staff. Cooney/Waters was named Healthcare Agency of the Year by the Holmes Report in April.
Creston CEO, Don Elgie, said: 'Cooney/Waters is an industry-leading firm with an excellent track record and a blue-chip client list.
'Cooney/Waters' strong cultural and strategic fit with Red Door, our UK healthcare PR firm, will enable seamless integration with Creston, which will open up new opportunities for growth in the US and Europe.'