Arena will begin working with the pharmaceutical and chemical companies, immediately, focusing on media planning and buying in multiple channels including content.
Arena’s first campaign for Merck Consumer Health is planned to launch in August 2014.
Last year, the company sponsored ITV’s national weather bulletins and created indents to promote various Seven Seas products.
Ashish Bhatt at Merck, director of marketing UK and Ireland, said: "As a portfolio brand we were looking for an agile, flexible and always-on approach, and Arena delivered all three, and much more besides.
"With a proven track record in supporting and promoting rapid growth, Arena will be a trusted partner as we expand existing brands and launch NPDs into the into the UK and Ireland markets."
Seven Seas works with TBWA\Manchester on its creative advertising and the Omnicom agency is not affected by the review.