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Diphenhydramine HCL

Respiratory Drugs
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Capsules , Pediatric Liquid, and Injectable
25 mg & 50 mg


An antihistamine that is an ethanolamine derivative, diphenhydramine competes with histamine at H1 receptor sites, and prevents but does not reverse histamine-mediated responses.

It provides local anesthesia, anticholinergic, and antitussive effects.

It is well absorbed orally and distributed in high levels to spleen, lungs, and brain. It is able to cross the placental barrier.

The drug is metabolized in the liver and excreted as metabolites in the urine.


Used in allergy symptoms and as a sedative for agitation in pruritus. It also helps to suppress cough reflex by its action on the brain.

Drug Interactions or Contraindications

Use cautiously with central nervous system depressants as it can increase sedative effect.

Adverse Reactions

CNS:  high sedative effect, poor coordination

CV:  palpitations

GI:  dry mouth, diarrhea, constipation

GU:  urine retention

Dosage Recommendations

1 mg/kg to 2 mg/kg, PO or SQ, q12hrs  34


2 mg/kg, PO or SQ, BID  12


5 mg/lb, PO or SQ, BID  20


10 mg/kg, BW , SQ  19


  • Can be purchased over the counter in tablet or topical form, or in combination with cough syrups.
  • Give with food to avoid GI reactions.
  • Store drug at room temperature.

Posted on June 23, 2003, 16:41, Last updated on May 27, 2012, 17:03 | Respiratory Drugs

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