It really depends on what has been eaten. First, a bit of Digestion 101. The gall bladder helps digest fats, but it also is a way for toxins to leave the body. So, part of the toxins going out through the bile are involved in fat digestion and absorption. This means the toxic products can come back into the body. (This very simplified explanation is called enterohepatic circulation.)
Activated charcoal works by grabbing onto the toxic thing in the intestinal tract and making it go out in the feces, rather than get (re-)absorbed. This means activated charcoal can work on anything that has its negative reaction through the intestines.
But, if the ingested item has already been absorbed and it doing its thing elsewhere in the body, the activated charcoal won’t prevent that. An example would be dogs who lick off over the counter (poisonous) flea medications and have seizures – these dogs need fluids and time to get the poison out of their body.
Your best bet – ask you veterinarian if it will help the particular situation your dog or cat is experiencing.