3. Rhubarb Leaves
Oxalic acid is the substance that makes rhubarb leaves toxic. The plant is not as harmless as you might thing so be careful with it. Oxalic acid builds stones in the kidneys so refrain from eating too much of the stalks and leaves. These parts of the plant are toxin both when raw and cooked. If eaten in large amount, rhubarb leaves cause vomiting, nausea, diarrhea and abdominal pain.