Hi, Colief is a brand of lactase enzyme drops that are added to infant formula or breast milk to help digest lactose, which may cause colic or digestive discomfort in some babies. The use of colief is generally considered safe, but as with any medication or supplement, there can be potential side effects. In the case of long-term use of colief, there is limited research on its safety, and it is not clear whether it can cause any long-term problems. However, colief is not intended for long-term use and should only be used as directed by a healthcare professional. Regarding your baby s grey stools, it is important to consult with your pediatrician to determine the underlying cause. Grey or clay-colored stools can be a sign of a problem with the liver or bile ducts, so it is important to have your baby evaluated to rule out any serious conditions. If your baby is experiencing constipation, your pediatrician may recommend changes to the diet, such as increasing fiber or fluid intake or trying a different type of formula. Soy-based formulas, such as Wysoy, can be an option for babies who are allergic or intolerant to cow s milk protein. However, soy-based formulas may also have their potential side effects, so it is important to discuss the pros and cons with your pediatrician. If you require a personalized response, kindly use the Ask A Doctor service and post a follow-up. Take care. Hope I have answered your question. Regards, Dr. Priyanka, General & Family Physician