Aloe vera should not be used orally for dogs (it's fine topically, like in grooming products).  
 
It is mildly to moderately toxic for dogs and can cause, diarrhea, vomiting, depression, anorexia, and in rare cases, tremors.     
Aloe Vera Plant and Dogs - Medicine Plant, True Aloe Toxic to Dogs and Cats   Quote:
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				Aloe vera, which is from the Aloaceae family, is a common  houseplant. Aloes contain anthraquinone glycosides which are purgatives  (medications that encourage bowel movements). When ingested, these  glycosides are metabolized by intestinal bacteria forming compounds that  increase mucus production and water in the colon. This can result in  vomiting and diarrhea. Other clinical signs seen with aloe vera  ingestion include depression, anorexia, changes in urine color, and  rarely, tremors.
			
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