Citrus and Dogs All citrus fruits contain organic chemical compounds called psoralens. These compounds are found throughout the lemon but are in highest concentrations in the skin, seeds, and the pith. Psoralens can cause an array of symptoms in dogs depending on the dog's exposure. Psoralens aren't just found in citrus, though. They can be in other plants such as celery, parsley, West Indian satinwood, and the common fig as well. An accidental lick won't be so detrimental to the health of your dog, but if even just one whole lemon were to be ingested, you may start to see clinical symptoms. If your dog were to eat a lemon, you may start to see signs of vomiting and diarrhea.