Pegasus' 100-strong team will retain its brand, and become part of Ashfield Healthcare Communications (AHC), part of UDG's largest division, Ashfield Commercial and Medical Services (ACMS). AHC, which incorporates previously acquired brands such as Galliard and iMed Comms, is led by CEO Viv Adshead.
UDG acquired the Brighton-based agency for £16.8m ($24.6m), with a £10.1m initial consideration followed by £6.7m deferred over a three-year earn-out.
It is the third healthcare comms M&A this week, after Chime-owned OPEN Health acquired UK-based international agency Choice Healthcare Solutions on Tuesday, and Omnicom bought American firm Rabin Martin on Monday.
Pegasus deputy managing director Simon Hackett steps up to the role of MD, which was previously held by Lisa Bradley, the firm's founder. Bradley becomes chair, succeeding Tim Adams – also the chair of Consolidated PR and founder of Lexis – who steps down.
Hackett will report to David Moore, director as AHC. Moore said the acquisition "broadens our capabilities in patient and consumer communications programmes, as well as corporate reputation and crisis management for our growing client base".
Hackett told PRWeek there would be no redundancies, and said: "While we will be keeping our name, location, great team and compelling mission of 'inspiring healthy decisions', all of which have been important ingredients in our success, we will enjoy the benefits of aligning with Ashfield in terms of service offering, talent, business development and global presence."
Pegasus primarily works in the UK market but also has European and global projects to its name. Clients include Bayer, GSK, Novo Nordisk and Unilever.
In UDG's interim results for the six months to 31 March 2016, published today, the company reported revenue from continuing operations of €472.4m (£362m, $530), of which €291.2m came from ACMS and nearly half of that from the UK.
In the PRWeek Global Agency Business Report, launched earlier this month, Pegasus was confirmed as the UK's largest outside of London, leapfrogging Scotland's The Big Partnership. It was also named the second largest healthcare agency in the UK, with 2015 revenue of £8.9m ($13m).