Common side effects of Benadryl include constipation, sedation, urinary retention, diarrhea, vomiting, increased heart rate, and loss of appetite for some dogs. Always be sure to monitor your dog closely when giving medication for the first time.
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Children's liquid Benadryl can be used for small dogs using the same dosage. Avoid the adult formula of liquid Benadryl, as this often contains alcohol, which is toxic to dogs.
If your veterinarian says it's okay, give your dog 2 mg of Benadryl per 1 kg (2.2 lb) of body weight, or follow your veterinarian's dosing suggestion. Repeat the dose every 4-6 hours while you need to keep your dog sedated. Do not exceed a dose of 4 mg of Benadryl per 1 kg (2.2 lb) of body weight every 4-6 hours.
There is no clear evidence that any one antihistamine works better than another, so other factors can be considered when choosing an allergy medication.