Quote -- "I have read/heard that a dog could eat an entire package of rat poison without harm."
Yikes - this is absolutely NOT true!! A lot depends on the type of rodenticide used. Some are quite lethal. Some info copied and pasted below:
Rodenticides
(from the NPCAA website)
Bromethalin
Cholecalciferol
Diphacinone
Fumarin
Strychnine
Warfarin
Zinc Phsphide
Poisons intended to kill rats, mice, gophers, moles and other mammalian pests are among the most common and deadly of small animal toxins. Substances highly poisonous to the pests are just as lethal to our pets. Rodenticides are highly toxic and any such poisons designed to kill small mammals need to be carefully contained in closed metal cabinets or high on stable shelving. The poisons usually come in flimsy cardboard containers and any dog, puppy or cat can chew through the container to get at the bait.
Rodenticides are classified according to both their basic ingredient compounds and by how they act on their target. These categories include: Anti-coagulant rodenticides, cholecalciferol, strychnine, zinc phosphide, bromethalin, compound 1080 and others.
The most common rodenticide poisoning seen in veterinary practice is that of the anti-coagulant rodenticides. These poisons - with ingredient names like warfarin, fumarin, diphacinone, bromadiolone - act by interfering with the animal's ability to utilize Vitamin K. One of they key roles of Vitamin K is in the production of coagulation factors in the body which cause blood to clot when necessary. Without the necessary coagulation factors, normal minor bleeding in the body goes unchecked which, without treatment, becomes major bleeding, with blood loss anemia, hemorrhage and death resulting. With most anti-coagulant rodenticides, signs are not seen until 3-5 days after the pet has ingested the poison. Clinical signs include weakness, difficult breathing, pale mucous membranes, and bleeding from the nose.
Other types of rodenticides have different mechanisms of action with some (i.e., strychnine and bromethalin) causing neurological signs such as incoordination, seizures and others cardiac failure (i.e., cholecalciferol). If accidental ingestion of rat poison is suspected, contact your veterinary clinic immediately, even if your dog or cat is showing no obvious signs of being ill. Be sure, if possible, to bring the poison container in to the clinic in order to determine the specific toxin and provide the best treatment. Early recognition is critical as some poisons, particularly the anti-coagulant rodenticides, can be successfully treated if the poisoning is caught early and treated appropriately.