Xylitol, a naturally-occurring sweetener, is believed to prevent tooth decay by inhibiting the growth of bacteria that cause cavities. Giving youngsters a syrup containing the sugar substitute may make them less likely to develop early childhood cavities known as baby bottle tooth decay or nursing cavities. Severe tooth decay in early childhood can become a lifelong problem. In a recent study, just 24.2 percent of 9 to 15 month olds receiving two doses of xylitol per day for nearly a year had tooth decay, compared with 51.7 percent in a control group.