There are a variety of over-the-counter constipation remedies for cats, including those containing magnesium. Magnesium is a natural laxative that can help to soften stools and make them easier to pass. However, it's important to speak with your veterinarian before giving your cat any type of laxative, as some can cause more harm than good if used improperly. If your cat is constipated, he or she may be uncomfortable and have difficulty passing stool. In severe cases, constipation can lead to an obstruction of the bowel, which can be life-threatening.