The catalyst for the acquisition was last year's $1.24 billion takeover by GSK of the US oral healthcare company Block Drug, which operates as Stafford-Miller in the UK, whose flagship brand is Sensodyne toothpaste.
Clive Holland, who heads Grey's global GSK business, said: "This deal gives GSK a real advantage in terms of controlling its advertising. Where possible, GSK will globalise copy."
PTK and its 11 staff will operate as a specialist unit at Grey's offices in London's Great Portland Street, where it will be known as Grey PTK. It will draw on the resources of the main agency and is expected to produce more creative work on its SKB brands for markets beyond the UK.
"We first looked at PTK some time ago but, because it was working for what was then an SKB competitor, we couldn't put a deal together," Holland said. "But within days of the Block deal, we were in touch again."
"PTK is a very experienced group of people and we admire the way they have been able to communicate just as well with consumers as health professionals,
Ian Hunter, PTK's chairman and managing partner, said: "It's a great match."