Pharmacological and Parenteral Therapies Q 88
A child weighing 30 kg arrives at the clinic with diffuse itching as the result of an allergic reaction to an insect bite. diphenhydramine (Benadryl) 25 mg 3 times a day is prescribed. The correct pediatric dose is 5 mg/kg/day. Which of the following best describes the prescribed drug dose?
A. It is the correct dose.
B. The dose is too low.
C. The dose is too high.
D. The dose should be increased or decreased, depending on the symptoms.
Correct Answer: B. The dose is too low.
This child weighs 30 kg, and the pediatric dose of diphenhydramine is 5 mg/kg/day (5 X 30 = 150/day). Therefore, the correct dose is 150 mg/day. Diphenhydramine, which is available as an over-the-counter medication, is a first-generation antihistamine that is used in a variety of conditions to treat and prevent dystonias, insomnia, pruritus, urticaria, vertigo, and motion sickness.
Option A: The correct dose is 150 mg/day. Diphenhydramine acts as an inverse agonist at the H1 receptor, thereby reversing the effects of histamine on capillaries, reducing allergic reaction symptoms. Given that diphenhydramine is a first-generation antihistamine, it readily crosses the blood-brain barrier and inversely agonizes the H1 CNS receptors, resulting in drowsiness, and suppressing the medullary cough center.
Option C: Divided into 3 doses per day, the child should receive 50 mg 3 times a day rather than 25 mg 3 times a day. Diphenhydramine overdose can cause significant toxicity, ranging from agitation to cardiac arrhythmias to rhabdomyolysis and classic anticholinergic toxidrome. Further studies are needed to investigate the potential treatment of diphenhydramine toxicity with the use of sodium bicarbonate and intravenous lipid emulsion therapy.
Option D: Dosage should not be titrated based on symptoms without consulting a physician. Diphenhydramine can be given by tablet, capsule, or in solution by mouth; by intramuscular (IM) or intravenous (IV) injection; or topically. Diphenhydramine is now available over the counter and it is important for the pharmacist and nurse practitioner to educate the patient on the safe use of this agent.