Common uses of Black walnut: intestinal parasites
The hulls of black walnuts are recommended for use as a deworming agent in pets. The active ingredients are purported to be tannins and alkaloids.

Therapeutic Uses for Black walnut
Black walnut has been used to treat dogs and cats with tapeworms.

There have been anecdotal reports of black walnut being effective against heartworms. However, there is no proof of this or any controlled studies backing up this claim. Some experienced holistic veterinarians use black walnut to first “weaken” the heartworms to make conventional therapy more effective, but once again controlled studies are lacking to support this claim. Heartworm prevention is preferred to treatment.

If your doctor is skilled at using black walnut, it can be used in an attempt to rid the pet of heartworms; however, in most if not all cases conventional therapy (perhaps also using herbs such as milk thistle to support the liver and other supplements to strengthen the heart) will need to be administered.

Black walnut Safety Issues
Black walnut is usually considered to be too toxic to use without veterinary supervision. There are reports of toxicity due to fungal contamination. The tannins and alkaloids may lead to vomiting and diarrhea. Most conventional dewormers (and other herbal deworming preparations) are much safer.