A new federal law requiring that sesame be listed as an allergen on food labels is having unintended consequences — increasing the number of products with the ingredient.

Food industry experts said the requirements are so stringent that many manufacturers, especially bakers, find it simpler and less expensive to add sesame to a product — and to label it — than to try to keep it away from other foods or equipment with sesame.

As a result, several companies — including national restaurant chains like Olive Garden, Wendy's and Chick-fil-A and bread makers that stock grocery shelves and serve scho...