At the standard concentration every 5 mL of liquid contains 12.5 mg of diphenhydramine and can be given to dogs at a dosage of 0.4 mL/lb. For reference a US teaspoon measurement is 5 mL.t the standard concentration every 5 mL of liquid contains 12.5 mg of diphenhydramine and can be given to dogs at a dosage of 0.4 mL/lb. For reference a US teaspoon measurement is 5 mL.