Dosing for Allergies or Cold Symptoms. The recommended diphenhydramine dosage for treating allergy or common cold symptoms in adults and children ages 12 years and older is 25 to 50 mg every four to six hours. For children ages 6 to 12 years old, the recommended dosage is 12.5 to 25 mg every four to six hours.Dosing for Allergic Reactions or Motion Sickness. The recommended diphenhydramine dosage for treating allergic reactions or motion sickness in adults is 25 to 50 mg every six to eight hours.or children ages 6 to 12 years old, the recommended dosage is 12.5 to 25 mg every four to six hours. Dosing for Allergic Reactions or Motion Sickness. The recommended diphenhydramine dosage for treating allergic reactions or motion sickness in adults is 25 to 50 mg every six to eight hours.