It's not safe for humans to ingest, so I would guess it's toxic to our pets, too.
BORIC ACID
Inhalation:
Causes irritation to the mucous membranes of the respiratory tract. May be absorbed from the mucous membranes, and depending on the amount of exposure could result in the development of nausea, vomiting, diarrhea, drowsiness, rash, headache, fall in body temperature, low blood pressure, renal injury, cyanosis, coma, and death.
Ingestion:
Symptoms parallel absorption via inhalation. Adult fatal dose reported at 5 to > 30 grams.