While not poisonous, large quantities of rhubarb, stems or leaves, offer possible risks, and the stems may be as bad as the leaves for susceptible people.
To complicate things, some rhubarb chemicals can have positive or negative effects.
In cell cultures using an extract of the entire plant, studies found some antimutagenic effects, depending on whether water or organic solvents were used.
Rhubarb also contains some chemicals called sennosides that are used as laxatives. Very large quantities can cause severe dehydration. The laxative effect may be where the idea of a poison arose, but the laxatives are in the stems, too.
When one sennoside, called emodin, is metabolized by the liver, one of the products, under alkaline conditions, has weak mutagenic effects, but in tests, emodin halted cancer cell growth in the test tube.
Rhubarb also contains oxalic acid, which is somewhat toxic. If there is a difference in the concentration in leaves and other parts, it is less significant than the fact that rhubarb has a high oxalic acid content overall.
Oxalic acid scavenges calcium and magnesium, and a buildup may be risky for people genetically predisposed to kidney stones.