It's a moment nearly everyone has experienced. You're contentedly chewing a wad of gum when an unforeseen turn brings about a quick disposal—the hard way. Whether the cause is imminent detection by a high school teacher, a dearth of garbage cans or even an untimely hiccup, you gulp down the rubbery gob whole. It's only then that a refrain from childhood echoes through your mind: "Don't swallow chewing gum—it will stay in your system for seven years!" As the minty mass descends into your digestive abyss, you wonder: "Will I really be part Wrigley for years to come?"

Rest assured—this decades-old bit of folklore, of unknown origin but almost universal renown, has little basis in fact. Asked if the rumor is medically unfounded, pediatric gastroenterologist David Milov of the Nemours Children's Clinic in Orlando, Fla., replies: "I can tell you that with complete certainty."

If the legend were true, Milov says, "that would mean that every single person who ever swallowed gum within the last seven years would have evidence of the gum in the digestive tract," but colonoscopies and capsule endoscopy procedures turn up no such evidence. "On occasion we'll see a piece of swallowed gum," he says, "but usually it's not something that's any more than a week old."

According to Rodger Liddle, a gastroenterologist at the Duke University School of Medicine, "nothing would reside that long, unless it was so large it couldn't get out of the stomach or it was trapped in the intestine." To put that size into perspective, Liddle says that swallowed quarters usually pass, but that larger coins or objects might not.

So what does become of gum that's been chewed up but not spit out? Not much, as it happens. Some of the components, such as sweeteners, are broken down, but the gum's base is largely indigestible. The Food and Drug Administration defines chewing gum base as a "nonnutritive masticatory substance" that may be composed of any number of natural or synthetic elastomers, or rubberlike materials, as well as plasticizing softeners, resins and preservative antioxidizing agents. The permitted elastomers include natural, tree-derived chicle, a gum chewed by indigenous Central Americans, and the somewhat less traditional butyl rubber, which also finds use in the manufacture of inner tubes.

Chewing gum, of course, has been around in one form or another for thousands of years: tooth-marked lumps of birch bark tar have been found in Europe that date back to the Mesolithic period of the Stone Age. And this past summer, researchers reported that quids—balls of plant material chewed by ancient Native Americans—had yielded DNA from members of a tribe called the Western Basketmakers, who lived in the southwestern U.S. some 2,000 years ago.

Unsurprisingly, perhaps, the human body cannot do much with these rubbery concoctions, resilient as they are. Chewing gum "is pretty immune to the digestive process," Milov says. "It probably passes through slower than most foodstuffs, but eventually the normal housekeeping waves in the digestive tract will sort of push it through, and it will come out pretty unmolested."

Nevertheless, the usually safe passage of gum through the system doesn't mean it is wise to habitually swallow it. As Milov and his colleagues wrote in Pediatrics in 1998, chronic gum swallowing—or swallowing gum in conjunction with other indigestibles—can spell trouble. The team's report describes three children suffering from gum-based gastrointestinal blockages, two of whom received gum as positive reinforcement for good behavior and regularly disposed of the treat by swallowing it. In both cases the children became constipated, as the gum snowballed into a substantial "taffylike" mass that required extraction. In the third patient, a girl just a year and a half old, four coins were found lodged in the esophagus, fused into a single blob by a wad of chewing gum.

"I've had another case that was really interesting," Milov adds, "and that was somebody who swallowed sunflower seeds—[and] also, the shells." Upon examining the patient's lower digestive tract, Milov found "all these very prickly seeds that were congealed around gum," forming a body that he describes as "like a porcupine."

Whereas the real (if remote) prospect of an internal quilling ought to be enough to discourage anyone from regularly swallowing gum, the mythical seven-year deterrent persists. Because it causes no real harm, and in fact probably serves to prevent many cases like those Milov describes, the urban legend seems likely to stick around for the foreseeable future. Unlike, thankfully, that wad of spearmint gum you swallowed in high school.