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The comparison of cola with tobacco, however, angers members of the National Soft Drink Association (NSDA), a trade group representing soft drink makers, marketers, and distributors.

"The author's personal conclusion that the industry's marketing efforts of products which contain caffeine 'strongly parallels' nicotine is irresponsible. Every major independent health organization, including the National Institutes of Health, the American Medical Association, [and] the U.S. Food and Drug Administration, tells consumers that moderate consumption of caffeine is safe," NSDA spokesman Jeff Nedelman is quoted as saying in a press release criticizing the study.

Griffiths tells WebMD, "Caffeine is the most widely consumed psychoactive -- mood altering -- drug in the world. At doses that are delivered in a can of soft drink, it produces mood and behavioral effects. Those mood effects are well known to most consumers of caffeine, and they include things like increased sense of well-being, increased alertness, concentration, some increased sociability, and decreased sleepiness."

The downside, however, is that daily consumption of caffeine in doses equivalent to two to three cans of soft drinks a day produces physical dependence, indicated by withdrawal symptoms when caffeine users try to go cold turkey, Griffiths adds. Characteristic symptoms of caffeine withdrawal include headache, tiredness, lethargy, and sometimes irritability; in extreme cases, people can have flu-like symptoms, nausea, and vomiting.

"We also know that the development of physical dependence is intimately linked to the continued daily administration of compounds that contain caffeine, so that most people who are regular consumers of caffeine are waking up in the morning and taking caffeine, experiencing the elevation in mood, and not recognizing that by not having caffeine overnight, they're actually in a low-grade withdrawal state. Caffeine is suppressing low-grade withdrawal symptoms, and it's that mechanism that is so important to the [regular use of the product]," Griffiths says.

In their study, Griffiths and Vernotica recruited volunteers who reported drinking cola at least once a week, and who expressed a preference for a specific brand of cola based on flavor and caffeine content. The prospective tasters were secretly screened for their ability to distinguish between regular and diet Coke; subjects who couldn't tell the difference (seven of 32) were then excluded.

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