Natural butter is gluten-free, but flavored butters and other options may contain additives that have gluten. Gluten is a wheat-based collection of proteins that holds food together. Butter is a dairy product that generally lacks this collection of proteins.
Celiac disease is the primary concern for people who consume foods containing gluten. It is an autoimmune disease that causes the body to attack the lining of the small intestine, even when small amounts of gluten are ingested. This attack leaves the body unable to absorb enough nutrients, which can lead to a host of medical problems. As of 2014, the only treatment is avoidance of foods that contain gluten.