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Hello, welcome to JustAnswer! I am a licensed veterinarian, and I'll be happy to help you in any way I can. The bottle of Benadryl should have all of the information you need to determine the proper dose. It should indicate how many mg are in each mL of liquid. Typically, most benadryl liquid preparations are 12.5mg per 5mL, so in order to administer 10mg, 4mL would be required (2.5mg per mL).