Wasabi is not poisonous to dogs nor is it toxic to dogs.
Although because wasabi is spicy it can cause stomach upset and gastrointestinal upset in dogs if eaten.
Wasabi also contains allyl isothiocyanate that can irritate a dogs digestive tract and cause symptoms such as diarrhea, abdominal pain and vomiting although your dog will likely be fine if they eat small amounts of wasabi but it's best to not feed it to them.
If the dog ate a small amount there's no need to take them to the vet unless the dog has eaten a huge amount of wasabi.
The wasabi is made from the grated rhizome of the wasabi plant also known as the Wasabia Japonica.
The Wasabi paste that is sold in many restaurants and in U.S. grocery stores is imitation wasabi and not real wasabi.
Imitation wasabi is made of horseradish, green food dye and mustard powder.
Imitation wasabi mimics the flavor and the color of real wasabi and is commonly used in Western restaurants.
Real wasabi is a root vegetable in the mustard family of Brassicaceae.
Real wasabi also has a potent flavor and aroma and is finely grated into a paste and uses an oroshigane and is also difficult to grow because real wasabi requires specific growing conditions.
Real wasabi is also expensive and loses it's intensity within 15 to 20 minutes of it being grated.
The real wasabi is also made of Japanese Horseradish and is the plant known as the Eutrema Japonicum or Wasabia Japonica.
Fake wasabi is instead made of European horseradish and often with green food coloring added to it.