Drug information of Diphenhydramin


Drug group:

 A histamine H1 antagonist used as an antiemetic, antitussive, for dermatoses and pruritus, for hypersensitivity reactions, as a hypnotic, an antiparkinson, and as an ingredient in common cold preparations. It has some undesired antimuscarinic and sedative effects.

Mechanism of effect

Histamine H1-receptor antagonist of effector cells in respiratory tract, blood vessels, and GI smooth muscle

Moderate to high anticholinergic and antiemetic properties


 Diphenhydramine is an antihistamine of the ethanolamine class. Ethanolamine antihistamines have significant antimuscarinic activity and produce marked sedation in most patients.

 In addition to the usual allergic symptoms, the drug also treats irritant cough and nausea, vomiting, and vertigo associated with motion sickness. It also is used commonly to treat drug-induced extrapyramidal symptoms as well as to treat mild cases of Parkinson's disease. Rather than preventing the release of histamine, as do cromolyn and nedocromil, diphenhydramine competes with free histamine for binding at HA-receptor sites.

Diphenhydramine competitively antagonizes the effects of histamine on HA-receptors in the GI tract, uterus, large blood vessels, and bronchial muscle. Ethanolamine derivatives have greater anticholinergic activity than do other antihistamines, which probably accounts for the antidyskinetic action of diphenhydramine. This anticholinergic action appears to be due to a central antimuscarinic effect, which also may be responsible for its antiemetic effects, although the exact mechanism is unknown.



Bioavailability: PO, 42-62% (drug is well absorbed but undergoes first-pass metabolism)

Onset: 15-30 min 

Duration: ≤12 hr (histamine-induced flare suppression); ≤10 hr (histamine-induced wheal suppression)

Peak serum time: 2 hr (PO)


Protein bound: 98.5%

Vd: 22 L/kg (Children); 17 L/kg (adults); 14 L/kg (elderly)


Metabolized by liver (first-pass)


Half-life: 5 hr (children); 9 hr (adults); 13.5 hr (elderly)

Excretion: Urine (50-75%), mainly as metabolites


Allergic Reaction

25-50 mg PO q6-8hr; not to exceed 300 mg/day

10-50 mg (no more than 100 mg) IV/IM q4-6hr; not to exceed 400 mg/day


50 mg PO 30 minutes before bedtime


25-50 mg PO q4hr PRN (syrup preferred); not to exceed 150 mg/day

Motion Sickness

Treatment or prophylaxis: 25-50 mg PO q6-8 hr

Alternatively, 10-50 mg/dose for treatment; may use up to 100 mg if needed; not to exceed 400 mg


25 mg PO q8hr initially, then 50 mg PO q6hr; not to exceed 300 mg/day

Alternatively, 10-50 mg IV at a rate not to exceed 25 mg/min; not to exceed 400 mg/day; may also administer 100 mg IM required

Drug contraindications

premature infants


  • May cause CNS depression, which can impair driving or operating heavy machinery
  • May potentiate effects of sedatives such as alcohol
  • Use caution in patients with angle-closure glaucoma, prostatic hypertrophy, stenosing peptic ulcer, pyloroduodenal obstruction, thyroid dysfunction
  • Elderly patients: Considered high-risk medication for this age group because it may increase risk of falls and has high incidence of anticholinergic effects; may exacerbate existing lower urinary tract conditions or benign prostatic hyperplasia; use in special situations may be appropriate; not recommended for treatment of insomnia, because tolerance develops and risk of anticholinergic effects increases

Points of recommendation

  • For motion sickness, take diphenhydramine 30 minutes before you will be in a situation that causes you motion sickness (such as a long car ride, airplane or boat travel, amusement park rides, etc). Continue taking diphenhydramine with meals and at bedtime for the rest of the time you will be in a motion-sickness situation.
  • As a sleep aid, take diphenhydramine within 30 minutes before bedtime.
  • Call your doctor if your symptoms do not improve after 7 days of treatment, or if you have a fever with a headache, cough, or skin rash.
  • This medication can affect the results of allergy skin tests. Tell any doctor who treats you that you are using diphenhydramine.
  • Do not give diphenhydramine to a child younger than 2 years old. Always ask a doctor before giving a cough or cold medicine to a child.Death can occur from the misuse of cough and cold medicines in very young children. You should not use diphenhydramine to make a child sleepy.

Pregnancy level


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