YES - It is very poisonous!
The modern rat poisons are much more toxic to dogs (and rats & mice of course) than the old warfarin based chemical poisons. They still work as an anti-coagulant (stops blood from clotting so that the animal dies of internal bleeding) but the modern baits are much more potent. The amount of bait required to poison your dog is small and the amount of antidote required to treat it is much higher.
Cats are less likely to be poisoned because they are fussy eaters and generally don't eat dead or sick mice or rats. But they can be poisoned too if they do eat any bait, and secondhand toxicity from eating poisoned vermin is rare but does occur.
WHAT DO I DO?
If you know your dog has eaten rat bait get to the vet ASAP! If it is recently ingested drugs can be given to make your dog vomit it up, and a blood test 72 hours later can tell us if the dog has absorbed any poison or not. It may be for these dogs, that no further treatment is required if clotting has not been affected. The PT blood test is done on-site at RVC on our IDEX machine with a result in an hour.
If the bait was ingested too long ago to prevent absorption, or your dog is already bleeding because of the poison, treatment with Vitamin K1 (by injection and then orally) will be started right away and continued for 3 weeks.
Severely affected dogs and cats sometimes require life-saving blood transfusion and hospital care.
DO NOT ALLOW DOGS or CATS to EVER HAVE ACCESS TO THIS POISON.