The Hollyoaks actress has promoted the first product from Natural Drinks Limited, which is a multivitamin drink.
"Vitzer is a girl's best friend; it not only tastes delicious but contains zero carbs, zero calories and zero sugar," Merna said.
"I keep one in my hand bag at all times for when I am in need of a serious refreshment session!"
She added: "Now for the first time there's also a drink I can stock in my fridge that not only I can enjoy, but my husband, who has type 1 diabetes can also love!"
Also joining the campaign is Australian Liverpool footballer Brad Jones.
Jones said: "For me it is essential to stay hydrated, coming out of the focus zone can allow for mistakes to happen, Vitzer enables me to keep ahead of the game and go the extra mile."