The agency has initially been tasked with helping the company expand Nature’s Bounty. It will also work on a new campaign for Solgar.
Havas Work Club will offer a "360 degree service" across all major markets, excluding the US and South America.
Paddy Griffith, the chief executive at Havas Work Club, said: "NBTY oversees a host of house-hold names and we’re looking forward to launching another one internationally.
"It’s an exciting brief that includes the flagship brand Solgar, so will provide us with an opportunity to think creatively about how we can leverage everything from digital channels, to experience design or concept stores to strengthen the portfolio on a global scale."