Side Effects of Dewormers

By Jane Meggitt

Routine deworming is part of good pet care. Fortunately, most dewormers eradicate parasites with minimal side effects, but some animals might experience gastrointestinal upsets or other reactions. Side effects of dewormers depend on the type of anthelmintic. While many dewormers are available over the counter, always check with your veterinarian before administering these medications to your pet. She can give you the proper dosage for your dog. If your dog has a reaction to the dewormer, call your vet.

Ivermectin for Heartworms

Unlike most other dewormers, heartworm medication requires a blood test to ensure your dog is heartworm-negative and is only available by prescription. If you give your pet an ivermectin-based heartworm dewormer, the medication will eradicate most other common worms, with the possible exception of tapeworms. Puppies over the age of 8 weeks can receive heartworm preventives. While side effects are uncommon, some dogs become lethargic, drool, shake or experience appetite loss, vomiting or diarrhea within 72 hours of receiving the medication.


Selamectin, a topical medication sold under the brand name Revolution, combines a heartworm dewormer with flea and tick control. It also requires a blood test and veterinary prescription. Puppies under the age of 6 weeks shouldn't receive selamectin. Hair loss can occur at the administration site. Side effects, while rare, are similar to those in ivermectin-based dewormers.

Pyrantel Pamoate

Pyrantel pamoate, sold in generic versions and under the brand names Nemex or Strongid, eradicates roundworms and hookworms. You must dose your dog twice, possibly three times, two to four weeks apart. If a dog has significant numbers of worms, the deworming process might cause diarrhea and occasionally vomiting. If a puppy or adult dog carries a huge parasite load, the sheer number of worms dying off at one time can result in impaction. The dead worms form an intestinal blockage. If your dog doesn't pass feces at his usual interval after deworming or appears to suffer from abdominal pain, take him to the vet immediately.

Praziquantel for Tapeworms

Praziquantel, marketed under the brand name Droncit, generally gets rid of tapeworms with the initial dose, although some dogs might require a second treatment if heavily infested. Since tapeworms are spread by fleas, you must follow the praziquantel administration with topical or oral flea control to prevent re-infestation. Available in several forms and often combined with other types of dewormers, praziquantel is usually well-tolerated although some dogs might become nauseous. If your vet gives your pet a praziquantel injection, the site is likely to sting. Puppies under the age of 4 weeks shouldn't receive praziquantel.


Fenbendazole, available in generic versions and marketed under the brand name Panacur, gets rid of hookworm and whipworms in canines, along with roundworms. Puppies over the age of 6 weeks can receive fenbendazole. This dewormers has very few side effects, although a small percentage of dogs might throw up after fenbendazole administration. Do not administer fenbendazole to any canine showing signs of illness.