Do not feed raw flax seed without exposing it to high heat, ie boiling it. It contains arsenates which are in the arsenic family of poisons. The heat destroys this property. Powdered flax is safe as it has been exposed to high heat.
"People who boil or soak their flax seed often do so because they believe that otherwise the flax might contain harmful levels of cyanide, which is toxic. In truth, while the seeds do contain the two components necessary to create cyanide, they are in different parts of the seed. Ironically, itís the contact with water that brings the two components together and causes the formation of cyanide. Soaking is actually the most dangerous way to feed flax."
Flax Seed: How and why to feed it | EQUINE Ink