Antidote will work on a campaign launching at the beginning of next year which will cover print, digital and social channels.
Henry Chilcott, the managing director at Antidote, said: "The sports nutrition market is enjoying huge growth as it expands into the broader, non-elite market.
"PhD are one of the most innovative and trusted brands in the category with ambitions to shake it up."
The brand previously handled its advertising in-house.
Jason Rickaby, the managing director at PhD, said: "Antidote displayed a brilliant strategic understanding of the market and some bold and creative ideas that demonstrated a healthy disrespect for the conventions of the market."