According to the Pet Poison Hotline, aloe vera has a mild to moderate toxic effect on cats. Aloe vera contains anthraquinone glycosides that are known to increase fluid in the intestines and cause a laxative effect. When ingested, aloe vera can cause vomiting, diarrhea and a change in urine color.
The ASPCA explains that aloe vera contains saponins that are toxic to cats and dogs. In addition to vomiting and diarrhea, aloe vera ingestion can cause depression, anorexia and tremors. The ASPCA advises pet owners to consult with a veterinarian or animal poison center immediately if their animals ingest aloe vera.