Packer Forbes provides numerous services including public relations, medical education, digital, patient advocacy and access comms to blue chip pharmaceuticals.
The transaction will see the agency rebrand itself as Four Health Communications. Alexa Forbes will remain as chief executive.
Four Health Communications will sit within the newly created Four Health division together with Four MSA. Now with 35 staff members in total, Four Health's integrated offering is expected to turn over £17m a year.
Nan Williams, Four Communications Group chief executive, said the acquisition of Packer Forbes would double the firm's presence in the health market. "They share our culture of delivering excellence for clients and creating an exciting place for staff to grow," Williams said.
This acquisition is the fourth by Four since it secured a £10m investment of equity finance from the Business Growth Fund in 2015.
Four has been expanding aggressively over the past five years, acquiring bgb in 2011, Colman Getty in 2012, Kinross & Render in 2013 and Consolidated PR in 2014. Last year, Four acquired MSA Media, Broadgate Mainland and Welsh integrated agency FBA.
After securing a further £5m of funding from HSBC in April 2016, Four plans more acquisitions this year.
Browne Jacobson advised Packer Forbes on the deal.
Four is not the only agency looking to strengthen its position in the healthcare market. In May, FTSE 250-listed healthcare outsourcer UDG Healthcare acquired Pegasus, the UK's largest comms firm outside of London.