Every part of annual vinca is poisonous. NC State Extension lists the flowers, fruits, leaves, roots, seeds, and stems as toxic (severity rated Low), with symptoms from eating large quantities including vomiting, diarrhea, low blood pressure, hallucinations, depression, tremors, seizures, coma, and death. The toxic compounds are the vinca alkaloids — the same class refined from this species into the chemotherapy drugs vincristine and vinblastine. NC State flags it as poisonous to humans and a problem for cats, dogs, and horses.
Toxic parts
flowers, fruits, leaves, roots, seeds, stems
Pet toxicity
NC State Extension lists Catharanthus roseus as a problem for cats, dogs, and horses. The vinca alkaloids it contains can cause vomiting, diarrhea, low blood pressure, tremors, and seizures.