• Benadryl can cause sleepiness in dogs as well as humans.woman and dog sleeping on bed image by nextrecord from Fotolia.com
  
  Benadryl, name brand diphenhydramine, is an antihistamine. Primarily used for allergies and inflammation, it also treats anxiety and vertigo. According to Vetinfo.com, the recommended dose for a dog is 1 mg per pound of body weight. Benadryl isn't recommended for dogs with glaucoma, hyperthyroidism, cardiovascular disease, lung disease or prostate enlargement. Use plain Benadryl; diphenhydramine is the active ingredient. Products containing acetaminophen (Tylenol) cause liver damage and possibly death. Some types contain alcohol, which also cause liver damage.
  
  

Drowsiness

• The most common side effect of Benadryl is drowsiness. Benadryl may cause the dog to fall asleep, or he may become lethargic. Since it causes sleepiness, Benadryl is not recommended for use with other medications that cause sedation, including tranquilizers and barbiturates.
  
  

Gastrointestinal

• Dogs might experience nausea and vomiting after a dose of Benadryl. The symptoms cease shortly after medication is stopped, if not before then. Dogs also might have diarrhea and decreased appetite.
  
  

Hyperactivity

• Some dogs experience hyperactivity on Benadryl. It can be combined with metoclopramide to reduce or eliminate this side effect.