Ketchum was awarded the account after a competitive pitch. It is believed the agency has been charged with handling global PR support for the drug firm's nutrition division.
Last year Pfizer bought rival pharma firm Wyeth in a deal worth £50bn. The acquisition included Wyeth's baby and toddler food business and its 90-year-old infant formula brand SMA Nutrition.
The promotion of infant formula has been embroiled in controversy for many years, with critics campaigning against firms encouraging new mothers to use their products over breast-feeding.
Last month, SMA Nutrition was rapped by the Advertising Standards Authority for a magazine ad that indirectly promoted the use of formula milk from birth.
The brand also came under fire in 2007 after pictures of model Katie Price feeding her new baby SMA baby milk appeared in OK! magazine. The next page of the magazine was given over to an ad for SMA formula designed for older infants.