Sugar-free sweets usually replace sugar with polyols ("sugar alcohols") such as maltitol, isomalt, sorbitol or xylitol, sometimes combined with intense sweeteners.
The warning on the pack
Products with more than 10% added polyols must say "excessive consumption may produce laxative effects". Take it seriously — eating a whole bag in one go is a common mistake.
Calories and teeth
Sugar-free does not mean calorie-free, though polyols contain fewer calories than sugar. They are kinder to teeth because mouth bacteria do not feed on them in the same way.
Keep xylitol away from dogs
Xylitol is very toxic to dogs, even in small amounts. Keep sugar-free sweets and gum out of reach of pets.