Benadryl is just one of the many antihistamine medications found in most homes throughout the country, but is probably the most common. Antihistamines are even used in pets to treat allergies, bee stings, and hives, but usually in much smaller doses than what you'll find in your family's medicine cabinet and, regardless, should only be given under the direct instruction of your veterinarian. While prescribed doses of antihistamines can be very safe, large doses can cause GI upset, lethargy, severe agitation or hyperactivity, and seizures. Even in small amounts, antihistamines can lead to increased vocalization, dilated pupils, nervousness, vomiting, and diarrhea. If your pet has accidentally ingested any amount of antihistamines and any of these signs are noted, potential overdose should be considered a possibility.