The win, which came without a pitch, follows the completion in April of Pharmacia's acquisition by Pfizer, maker of Viagra, for $60bn (£37.4bn). Both companies are now operating under the Pfizer name.
The account, which is understood to have been worth $147m in billings last year, according to TNS Media Intelligence/CMR, was previously held by Grey Global Group-owned MediaCom.
Carat won the estimated $700m consolidated Pfizer media planning and buying account in 2001, following a two-way fight against MindShare USA.
The media consolidation was sparked by Pfizer's $90bn acquisition of rival Warner-Lambert in 2000.
Carat handles some of the most successful drug brands in the world including Viagra, Listerine, Benadryl, Neosporin, hair-loss treatment Rogaine, known as Regaine in the UK, and Nicorette.
Pharmacia's UK media planning and buying account is held by OMD.
The news follows yesterday's announcement that Pharmacia chief financial officer Christopher Coughlin has moved to Interpublic Group to become chief operating officer.
The Interpublic post has been vacant for two years and comes as the advertising network strengthens its management, after it demoted chairman and CEO John Dooner to chairman and CEO of McCann-Erickson WorldGroup in February.
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