What is the maximum dose of diphenhydramine for allergic reactions?

• A. 25 mg
• B. 50 mg
• C. 75 mg
• D. 100 mg

Answer: (B)

Diphenhydramine relieves allergic symptoms by blocking histamine at H1 receptors, helping with itching, hives, and sneezing. In adults, it’s commonly given as 25–50 mg every 4–6 hours as needed, with 50 mg serving as the practical maximum per dose to keep side effects manageable. Exceeding 50 mg at a single dose increases the risk of drowsiness, confusion, dry mouth, urinary retention, and other anticholinergic effects. So, the highest dose you would administer at one time is 50 mg, while 25 mg is a valid lower dose and 75 mg or 100 mg would exceed the safe per‑dose limit. Keep in mind diphenhydramine is supportive in allergic reactions; epinephrine remains the primary treatment for more serious reactions.