No, it's not. I started giving CBD for Buttercup's epilepsy at my vet's advice (in addition to the prescription drugs she's on). There's a vet named Dr. Robert Silver who practices in Colorado where marijuana is legal. He 's written extensively about medical marijuana for both dogs and cats. He's published guidelines for administering CBD both with and without significant amounts of THC and it's what I've relied on when giving it to Buttercup. There CAN be adverse effects to giving too MUCH THC, so you DO need to be careful about the dosing, however. I should mention, also, that I live in Nevada, where marijuana is legal, so I have access to products with THC. Most anything you buy online or in most states will not have enough THC in it to worry about.
This PDF is what I have relied on for dosing Buttercup with CBD. I belong to a couple of CBD for canine epilepsy groups on Facebook and this handout is pretty much the Bible when it comes to figuring out dosing. http://hawthornevet.com/wp-content/uploads/2015/03/Medical-Marijuana-and-Your-Pet.pdf