In January, a new drink hit the shelves of Irish shops and supermarkets. It now jostles for position alongside an extremely similar, long-established and very popular product - from the same company. If you read the nutritional information on the back of both products, they are virtually indistinguishable. The ingredients are all but identical, and they both claim to be sugar free and have just two calories.

So why would Coca-Cola launch a new drink that seems to compete directly with one of own existing brands?

The product in question is Coca-Cola Zero, or "bloke Coke" as it has been dubbed. While Diet Coke may seem to have a unisex appeal, 80 per cent of its sales are to women. This is partly due to the fact that, until recently, dieting was a female domain. But it also dates back to the iconic "Diet Coke break" television advertisements which ran more than a decade ago, featuring a scantily-clad "Diet Coke hunk".

According to Sharon Walsh, brand manager of Coca-Cola Zero in Ireland, Coca-Cola identified a growing demand among men for a diet drink which still tasted like the original "red Coke". As any Diet Coke connoisseur will tell you, the flavour of the diet version is easily distinguishable from the original drink.

"What we've been working on for the last number of years is a low-calorie, sugar-free Coke brand that would be very similar to red/classic Coke," says Walsh.

"The reality is that consumers - predominantly men - love the taste of 'red Coke' but they've been looking for a low-cal alternative, because in more recent years, guys . . . are now more conscious about their health, their weight and how they look."

One of the few differences between the two is taste, but the key difference is how they are marketed. "We knew from doing research that the biggest thing to tell people is what [ Coca-Cola Zero] is, so that they understand the difference between this and Diet Coke," Walsh says.

A distinctly male-oriented television campaign for the new drink began last month, incorporating a "no downsides" theme, ie why can't all the great things life in life come without downsides? Some of the phrases used in Coke Zero advertisements in other countries have sparked controversy. In particular, the comparison drawn between the drink and "girlfriends without a five-year plan", or "stag nights without getting married", have attracted the wrath of feminist bloggers.

"The initial things are to drive awareness and drive trial - they're the two biggest objectives of any new product launch," Walsh says of the Irish marketing launch, on which more than €2 million has been spent.

As well as the television advertisements and an outdoor marketing campaign which hammered home the message, "Great Coke taste, zero sugar", a huge sampling campaign was carried out. This involved handing out free 150ml cans to one million people in Ireland - at construction sites, Croke Park and other places where Coke Zero's target audience can be found.

Coca-Cola is aware that the new brand will probably "cannibalise", or steal market share, from Diet and classic Coke. "We have to bear that in mind," Walsh says. However, she expects that the overall effect of the new brand will be the growth of the Coca-Cola category as a whole.

"We would have done extensive analysis into that prior to launching," she says. "By having three Cokes in the marketplace together, we are now able to offer a range of choices to everybody."

The process of researching, creating, launching and marketing a new product such as Coca-Cola Zero is known as "new product development" (NPD) - a notoriously risky undertaking, with 40 per cent of products failing at the launch stage.

Market research, and the identification of changes in consumer demands, is critical to the success of a new product. For example, Cadbury Ireland recently tapped into the growing trend of "in-home evening entertainment", such as having a girls' nights in. This niche was targeted with the development of Cadbury Snaps, "individual chocolate curves", which are ideal for sharing.

But although companies must respond to shifts in consumer demands, tinkering with a tried-and-tested formula can be a recipe for disaster. Back in 1985, Coca-Cola decided to alter its formula and launched "new Coke".

This move is now widely referred to as one of the biggest marketing flops in modern business history, as the company had to bow to huge pressure from Coke loyalists and reintroduce the original formula.

Speculation at the time suggested that Coca-Cola had planned the whole thing as a marketing stunt to revive interest in the drink. However, then company president Donald Keough said: "Some critics will say Coca-Cola made a marketing mistake. Some cynics will say that we planned the whole thing. The truth is we are not that dumb and we are not that smart."