Rhubarb contains oxalate, which have been reported to cause poisoning when large quantities of raw or cooked leaves are ingested.

The poison in rhubarb

Oxalates are contained in all parts of rhubarb plants, especially in the green leaves. There is some evidence that anthraquinone glycosides are also present and may be partly responsible. It is not clear as to the exact source of poisoning from rhubarb, possibly a result of both compounds. The stalks contain low levels of oxalates, so this does not cause problems.