https://phys.org/news/2018-11-veterinary-experts-cannabis-fatal-poison.html

It takes only a small amount to cause symptoms of poisoning in a dog or cat, said University of Alberta animal science instructor Connie Varnhagen.

"Dogs have so many more cannabinoid receptors in their brain and throughout their body," she said, and that sensitive network can simply be overwhelmed by THC, the psychoactive cannabinoid in marijuana.