Do not induce vomiting without first consulting your veterinarian or Poison Control center. Some toxins are caustic, and vomiting will only increase the damage.
Do not induce vomiting or give charcoal by mouth if your dog is severely depressed, comatose, unable to swallow or experiencing seizures.
It is possible to induce vomiting using hydrogen peroxide given in small amounts until the dog vomits or by using syrup of ipecac. It takes about 1 teaspoon of ipecac syrup per 10 pounds of body weight (on the average) to induce vomiting. It can be hard to give the proper dosage of ipecac syrup and an over dosage can cause heart problems, so it must be measured carefully. Apomorphine is a good choice for inducing vomiting but usually it is only available through your vet.