Use of GLP-1 receptor agonist medications (GLP-1s) has skyrocketed in recent years, and many people are using "compounded" GLP-1 products: medicines that include GLP-1s as well as other active substances, some of which are not regulated by the FDA. A new study found that compounded GLP-1s now make up a large majority of weight-loss medicines, commonly containing vitamin B12 analogs that have not been tested for safety in combination with GLP-1s.