Behavior

• Before a dog shows outward symptoms of illness, it may display behavior changes. If your dog has developed onion toxicity, it may become depressed or lethargic. It may sleep more, move more slowly and seem generally weakened.
  
  

Respiratory

• Respiratory symptoms are also common in dogs with onion toxicity. If your dog has onion toxicity, you may notice it is experiencing shortness of breath. A change in respiratory rate is also possible, so your dog may have an increase, or decrease, in its normal pulse: 70 to 120 beats per minute for a large dog larger than 50 lbs., 80 to 120 bpm for a 25 to 50 lb. dog and 90 to 160 bpm dogs less than 25 lbs.
  
  

Gastrointestinal

• Gastrointestinal symptoms are common in dogs with onion toxicity. If your dog has onion toxicity it may have diarrhea. Your dog may also vomit if it has onion toxicity. An upset stomach can cause your dog to lose its appetite and avoid eating.
  
  

Miscellaneous

• There is a variety of other symptoms your dog may experience if it develops onion toxicity. You may notice blood in your dog's urine. Your dog may also become jaundiced as a result of onion toxicity. Signs of jaundice include yellowing of the eyes, gums and skin.