Fenbendazole

Fenbendazole is a benzimidazole-based dewormer for dogs, cats, horses, and cows. This medication is a top pick among pet parents since it's relatively inexpensive, available over-the-counter, and has a long shelf life when stored correctly. 

Fenbendazole kills many common parasitic nematodes that infect canines like whipworms, lungworms, giardia, roundworms, and certain species of tapeworms. This medication attacks the internal structure of these parasites, which renders them incapable of feeding or reproducing.  

Many vets recommend this drug over other commercial dewormers since it's not as harsh on the intestinal tract as some others on the market. Fenbendazole is the active ingredient of Panacur and Safe-Guard but it's also found in many combination antiparasitics like Fentol Plus. 

Dosage

A dosage of 22.7 mg per lb of bodyweight is usually recommended for 3 consecutive days for dogs with an active parasitic infection. This medication comes in several strengths and formulations for dogs from 5 lbs and up.

2.5% fenbendazole liquid suspension:

• 0.9 ml per lb for dogs under 6 months
• 1.82 ml per lb for dogs 6 months or older

10% fenbendazole liquid suspension:

• 0.23 ml per lb for dogs under 6 months
• 0.45 ml per lb for dogs 6 months and older

Granules: 

• 1 gram per 10 lbs of body weight

Oral paste:

• 2 syringe graduations per 2.2 lbs/1kg

Panacur Favourites Tablets:

• 1 tablet per 10 lbs of weight

*This medication is available for canines in oral paste and tablet form in the UK and Canada. Fenbendazole paste is only sold for equine animals in the USA. 

Dosage instructions

Most versions of this drug recommend owners pour the medication over the pet’s food. Besides making it easier to dose your dog, mixing fenbendazole with food will minimize nausea and help your pet keep it down. 

Efficacy

The College of Veterinary Medicine at Cornell conducted a controlled study on the use of fenbendazole as a treatment for dogs with giardia, a particularly resilient parasite. Stool samples were taken from each of the 6 study groups at the end of the study and found no evidence of giardia infestation in the dogs treated with fenbendazole. 

A study comparing piperazine and fenbendazole found that fenbendazole was effective at killing most whipworm larvae in canines, whereas piperazine, had little effect against the larval stages. This study concluded that fenbendazole was 100% successful at eliminating whipworm larvae and killed most but not all of the older life stages of Toxocara and Toxascaris (two species of roundworms).

Side effects

• Nausea

• Stomach upset

• Diarrhea

• Sluggishness

Considerations

Be very careful when handling your dog's stools before and after treatment with any antiparasitic. Many types of worms that canines carry can infect humans too. After disposing of your dog's stool, thoroughly sanitize your hands with hot, soapy water.

Drug interactions

Fenbendazole is shown to have adverse reactions when combined with glucocorticoids and some types of dewormers in animal models. Ask your vet before giving your fenbendazole if your pet is on any of the following:

• Praziquantel

• Dexamethasone

• Salicylanilides

• Niclosamide 

• Other dewormers

Allergic reactions and sensitivity

Allergic reactions to fenbendazole are uncommon; however, some pets exhibit signs similar to a drug allergy in response to compounds excreted by the parasites as they die. 

Don’t give this medication to dogs who have had a reaction to benzimidazole-based medications in the past. The most commonly used benzimidazoles for dogs are:

• Oxfendazole

• Mebendazole

• Febantel

• Oxibendazole

• Flubendazole 

Frequently asked questions

How do I know if my dog has worms?

Some indications your dog may have worms are swelling of the abdomen, appetite changes, weight loss, pale gums, lethargy, and the appearance of parasites in the excrement. Unfortunately, you can't rely on visual cues alone. Many species of parasites can linger in an animal's system for months before they become active and multiply. 

Is fenbendazole safe for 4-week-old puppies? 

No. Only treat puppies with this medication if they're older than 6 weeks. 

Is it normal to see worms in the feces after giving fenbendazole?  

As gross as it is, seeing worms in your dog's excrement is a sign that fenbendazole is doing its job. If you are still seeing parasites in Fido's stool a week or more after treatment, talk to your vet to see if you need to give them another round of dewormer. 

My dog had diarrhea after taking fenbendazole, is this a side effect or a reaction?

Unfortunately, frequent loose stools are a side effect of the body expelling the parasites killed by fenbendazole. Diarrhea typically subsides within two days of the last dose. If your dog has diarrhea for more than 48 hours after treatment, contact their vet to see if something else is going on. 

Will fenbendazole kill heartworms?

No. Fenbendazole is neither a heartworm preventative nor treatment. 

Can I give fenbendazole for pregnant dogs? 

This medication is safe for canines that are pregnant or nursing, though a lower dosage is required. Dosage guidelines for pregnant dogs state they should take no more than 11.3 mg per pound of body weight.