For more than a century, soft drink manufacturers have been well aware that it helps to add an addictive drug to get customers hooked on their product. It is many years since cocaine has been added to colas as a matter of practice. Now it has become standard to add caffeine. Caffeine alone has an unattractively bitter taste but this is always masked in a soft drink by mixing in sugar or an artificial sweetener, plus some flavouring such as citric acid for a lemon flavour.

Caffeine can have subtle effects in a drink. A recent experimental study at the University of Sussex in Britain looked at how a liking for particular flavoured drinks can develop. Participants were given two types of drink to sample, both with distinctive but novel flavours. Only one contained caffeine - at a much lower concentration than that of many of the so-called ''energy drinks'' commercially available today.

After about a week, participants were asked to rate how much they liked each drink. The one containing caffeine was given much higher ratings, even though the participants could not tell which drink contained caffeine. Adding caffeine had conditioned an increased liking for the associated flavour, without the participants being aware this was happening.

Until recently, the concentration of caffeine in soft drinks remained relatively low. The caffeine content of most colas today is about the same as that of an average cup of tea.

But with the advent of ''energy drinks'' there has been a progressive rise in concentration.