Sugar-free gum sweetened with xylitol has been shown to reduce cavities and plaque. The sweetener sorbitol has the same benefit, but is only about one-third as effective as xylitol. Other sugar substitutes, such as maltitol, aspartame and acesulfame K, have also been found to not cause tooth decay. Xylitol is specific in its inhibition of "Streptococcus mutans", bacteria that are significant contributors to tooth decay. Xylitol inhibits "Streptococcus mutans" in the presence of other sugars, with the exception of fructose. Xylitol is a safe sweetener that benefits teeth and saliva production because, unlike most sugars, it is not fermented to acid. Daily doses of xylitol below 3.44 grams are ineffective and doses above 10.32 grams show no additional benefit. Other active ingredients in chewing gum include fluoride, which strengthens tooth enamel, and p-chlorbenzyl-4-methylbenzylpiperazine, which prevents travel sickness. Chewing gum also increases saliva production.