Histantil , Phenergan
Classifications: antiemetic; antivertigo agent; phenothiazine;
Therapeutic: antiemetic
; antivertigo
Prototype: Prochlorperazine
Pregnancy Category: C


12.5 mg, 25 mg, 50 mg tablets; 6.25 mg/5 mL syrup; 12.5 mg, 25 mg, 50 mg suppositories; 25 mg/mL, 50 mg/mL injection


In common with other antihistamines, exerts anti-serotonin, anticholinergic, and local anesthetic action. Antiemetic action thought to be due to depression of CTZ in medulla.

Therapeutic Effect

Long-acting derivative of phenothiazine with marked antihistamine activity and prominent sedative, amnesic, antiemetic, and anti–motion-sickness actions.


Symptomatic relief of various allergic conditions, to ameliorate and prevent reactions to blood and plasma, and in prophylaxis and treatment of motion sickness, nausea, and vomiting. Preoperative, postoperative, and obstetric sedation and as adjunct to analgesics for control of pain.


Hypersensitivity to phenothiazines; acute MI; angina, atrial fibrillation, atrial flutter, cardiac arrhythmias, cardiomyopathy uncontrolled hypertension; MAOI therapy; comatose or severely depressed states; children with Reye's syndrome, hepatic encephalopathy, hepatic diseases; acutely ill or dehydrated children; children <2 y; pregnancy (category C), lactation, newborn or premature infants.

Cautious Use

Impaired liver function; epilepsy; bone marrow depression; cardiovascular disease; peripheral vascular disease; asthma; acute or chronic respiratory impairment (particularly in children); hypertension; narrow angle glaucoma; stenosing peptic ulcer, pyloroduodenal obstruction; prostatic hypertrophy; bladder neck obstruction; older adult or debilitated patients; children >2 y.

Route & Dosage

Motion Sickness
Adult: PO/PR 25 mg q12h prn
Child (>2 y): PO/PR 10.5 mg/kg/dose q12h prn (max: 25 mg/dose)

Adult: PO/PR/IM/IV 12.5–25 mg q4–6h prn
Child (>2 y): PO/PR/IM/IV 0.25–0.5 mg/kg q4–6h prn (max: 25 mg/dose)

Adult: PO/PR 12.5 mg q.i.d. or 25 mg h.s. IM/IV 25 mg, repeat in 2 h if necessary, switch to PO
Child (>2 y): PO 0.1 mg/kg q6h and 0.05 mg/kg at bedtime prn

Adult: PO/PR/IM/IV 25–50 mg/dose
Child (>2 y): PO/PR/IM/IV 12.5–25 mg/dose (max: 50 mg)


  • Give with food, milk, or a full glass of water may minimize GI distress.
  • Tablets may be crushed and mixed with water or food before swallowing.
  • Oral doses for allergy are generally prescribed before meals and on retiring or as single dose at bedtime.
  • Give IM injection deep into large muscle mass. Aspirate carefully before injecting drug. Intraarterial injection can cause arterial or arteriolar spasm, with resultant gangrene. Subcutaneous injection (also contraindicated) can cause chemical irritation and necrosis. Rotate injection sites and observe daily.

PREPARE: Direct: ??The 25 mg/mL concentration may be given undiluted. Dilute the 50 mg/mL concentration in NS to yield no more than 25 mg/mL (e.g., diluting the 50 mg/mL concentration in 4 mL yields 10 mg/mL).??Inspect parenteral drug before preparation. Discard if it is darkened or contains precipitate. 

ADMINISTER: Direct: Give each 25 mg or fraction thereof over at least 1 min.  

INCOMPATIBILITIES Solution/additive: Aminophylline, ampicillin, carbenicillin, cefazolin, cefotetan, ceftizoxime, chloramphenicol, chlordiazepoxide, chlorothiazide, dexamethasone, dimenhydrinate. furosemide, heparin, hydrocortisone, ketorolac, methicillin, methohexital, nalbuphine, nitrofurantoin, penicillin G sodium, pentobarbital, phenobarbital, thiopental. Y-site: Aldesleukin, allopurinol, amphotericin B cholesteryl complex, cefepime, cefmetazole, cefoperazone, cefotetan, doxorubicin liposome, foscarnet, furosemide, heparin, methotrexate, piperacillin/tazobactam, TPN.

  • Store at 15°–30° C (59°–86° F) in tight, light-resistant container unless otherwise directed.

Adverse Effects (≥1%)

Body as a Whole: Deep sleep, coma, convulsions, cardiorespiratory symptoms, extrapyramidal reactions, nightmares (in children), CNS stimulation, abnormal movements. Respiratory: Irregular respirations, respiratory depression, apnea. CNS: Sedation drowsiness, confusion, dizziness, disturbed coordination, restlessness, tremors. CV: Transient mild hypotension or hypertension. GI: Anorexia, nausea, vomiting, constipation. Hematologic: Leukopenia, agranulocytosis. Special Senses: Blurred vision, dry mouth, nose, or throat. Skin: Photosensitivity. Urogenital: Urinary retention.

Diagnostic Test Interference

May interfere with blood grouping in ABO system and may produce false results with urinary pregnancy tests (Gravindex, false-positive; Prepurex and Dap tests, false-negative). Promethazine can cause significant alterations of flare response in intradermal allergen tests if performed within 4 d of patient receiving promethazine.


Drug: Alcohol and other cns depressants add to CNS depression and anticholinergic effects.


Absorption: Readily from GI tract. Onset: 20 min PO/PR/IM; 5 min IV. Duration: 2–8 h. Distribution: Crosses placenta. Metabolism: In liver (CYP2D6, 2B6). Elimination: Slowly in urine and feces.

Nursing Implications

Assessment & Drug Effects

  • Supervise ambulation. Promethazine sometimes produces marked sedation and dizziness.
  • Be aware that antiemetic action may mask symptoms of unrecognized disease and signs of drug overdosage as well as dizziness, vertigo, or tinnitus associated with toxic doses of aspirin or other ototoxic drugs.
  • Patients in pain may develop involuntary (athetoid) movements of upper extremities following parenteral administration. These symptoms usually disappear after pain is controlled.
  • Monitor respiratory function in patients with respiratory problems, particularly children. Drug may suppress cough reflex and cause thickening of bronchial secretions.

Patient & Family Education

  • For motion sickness: Take initial dose 30–60 min before anticipated travel and repeat at 8–12 h intervals if necessary. For duration of journey, repeat dose on arising and again at evening meal.
  • Do not drive or engage in other potentially hazardous activities requiring mental alertness and normal reaction time until response to drug is known.
  • Avoid sunlamps or prolonged exposure to sunlight. Use sunscreen lotion during initial drug therapy.
  • Do not take OTC medications without physician's approval.
  • Avoid alcohol and other CNS depressants.
  • Relieve dry mouth by frequent rinses with water or by increasing noncaloric fluid intake (if allowed), chewing sugarless gum, or sucking hard candy. If these measures fail, add a saliva substitute (e.g., Moi-Stir, Orex, Xero-Lube).

Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug

© 2006-2023 Last Updated On: 01/27/2023 (0.02)
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