It seems like common sense that foods labeled “sugar free” would have no effect on sugar levels in the blood. But sometimes they do.

Most artificial sweeteners — saccharin, aspartame and sucralose, for example — offer the sweetness of sugar without the calories. They contain no carbohydrates, and so have no effect on blood sugar. But these sweeteners are sometimes paired in “sugar free” products with another sugar substitute called sugar alcohols.

Sugar alcohols get their name from their structure, which looks like a cross between a molecule of alcohol and sugar but is technically neither. Companies have added them to more and more “sugar free” products, like cookies, chewing gum, hard candy and chocolate. For people trying to manage their blood sugar, this can make interpreting nutritional labels a little tricky. While sugar alcohols provide fewer calories than regular sugar — in general about 1.5 to 3 calories per gram, compared with 4 calories per gram of sugar — they can still slightly raise your blood sugar.

According to health authorities, one way to account for them is to count half the grams of sugar alcohol in a product as carbohydrates, since roughly half of the sugar alcohol content is actually digested. You can identify sugar alcohols in an ingredient list by looking for words that end in “-ol,” like sorbitol, maltitol and xylitol. And in foods labeled “sugar free” or “no added sugar,” the precise sugar alcohol count must be listed separately under the nutritional information.