Deworming medications, also called anthelmintics, are generally safe to use in healthy animals. However, some dogs may experience negative side effects, especially if the worm infection is severe. Severe side effects from anthelmintics are an uncommon occurrence.
Mild Side Effects
Diarrhoea and vomiting are the most common side effects of deworming. Your dog may exhibit signs of lethargy and may not show much interest in food.
In cases of heavy worm infestations, a dog may develop an impaction as the dead or dying worms clog up the intestinal tract. This type of side effect is rare and is most often seen in very small dogs.
Some dogs may be allergic to deworming drugs, but allergies to anthelmintics are rare.
Passing the Worms
Because deworming drugs often work by paralysing the worms so that they lose their grip on the walls of the intestines, you may see live worms in your dog's stool. Depending on the type of worm, they may be long and might be moving, but will soon die once they are outside of your dog.
Your dog should be taken to the vet immediately if he has ingested an overdose of the drug. Overdose symptoms are drooling, weakness, twitching, staggering, head pressing and paralysis.