Persin

Persin is a fungicidal toxin present in the avocado. Persin is an oil-soluble compound structurally similar to a fatty acid, and it leaches into the body of the fruit from the seeds. 
The relatively low concentrations of persin in the ripe pulp of the avocado fruit is generally considered harmless to humans. Negative effects in humans are primarily in allergic individuals. When persin is consumed by domestic animals through the leaves or bark of the avocado tree, or skins and seeds of the avocado fruit, it is toxic and dangerous.
Pathology.
Consumption of the leaves and bark of the avocado tree, or the skin and pit of the avocado fruit have been shown to have the following effects:
Additional pharmacology.
Animal studies show that exposure to persin leads to apoptosis in certain types of breast cancer cells. It has also been shown to enhance the cytotoxic effect of tamoxifen "in vitro". Persin is however highly insoluble in aqueous solutions and more research will be needed to put it into a soluble tablet form.