Diphenhydramine is frequently ordered for cancer patients to aid in controlling nausea and itching. It may be given after a blood transfusion to limit allergic reactions to blood products. Because of its sedating properties, diphenhydramine is often used to assist in inducing sleep. It is also used to control nausea, treat the stiffness and tremor of Parkinson's disease, and control symptoms of extrapyramidal neurologic movement disorders (tremors and abnormal involuntary movements of the muscles) caused by some drugs used to treat psychosis or nausea and vomiting). The drug may also be formulated as a syrup and used to relieve a cough caused by minor throat irritation due to a cold or hay fever.
Diphenhydramine is an antihistamine that dries, sedates, and is distributed throughout the body. It is readily absorbed when taken by mouth, with peak action occurring about one hour after ingestion. The effects last from four to six hours. This type of drug seems to compete with histamine for receptor sites after exposure to an allergen. By blocking histamine from attaching to the receptor site, the drug decreases itchiness, a runny nose, hives, and other symptoms of an allergic reaction.
The dose should be adjusted depending on the needs of the patient and their response to the medication. Adults generally take from 25 mg to 50 mg, three to four times daily. For sleep, 50 mg at bedtime is the usual
Children weighing more than 20 pounds may take from 12.5 mg to 25 mg, three to four times daily. Children should not consume more than 300 mg in one day. The doctor may calculate a recommended dosage based on the child's weight. Parents should not double up on doses if one is missed.
Lotions or creams with diphenhydramine may be applied to the skin to relieve itching in adults and children older than two. The creams contain 1% or 2% diphenhydramine and may be used on the affected area three to four times per day. Topical diphenhydramine should not be applied to large areas of the body, blistered or oozing skin, sunburn, or lesions caused by poison ivy or chicken-pox. Patients should not use topical diphenhydramine with other antihistamine-containing lotions or creams.
Patients with angle closure glaucoma, peptic ulcer disease, bowel obstructions, an enlarged prostate, or difficulty urinating due to a blockage in the bladder should not use this medication without a doctor's order and monitoring. This drug should be used with caution in patients with asthma, heart disease, high blood pressure, or an overactive thyroid. Prior to taking this medication, patients with these conditions should discuss this medication with their doctor. Patients should not take diphenhydramine for several days prior to an allergy test. It will interfere with obtaining accurate results.
Elderly patients are prone to diphenhydramine's sedating effects. The drug may also cause dizziness and lower blood pressure in this population group. Patients should slowly change position from sitting or lying to standing when taking this medication.
Children also may experience drowsiness. In young children, this drug may produce the opposite effect. Pregnant women and those breast feeding should discuss the use of this and other drugs with their physician prior to use.
Drowsiness commonly occurs after taking diphenhydramine. This effect may be more pronounced if alcohol or another central nervous system depressant, such as a tranquilizer or pain medication, is also ingested. Those taking the drug should refrain from driving or operating machinery or appliances until the medication has worn off. It also may cause dizziness, coordination difficulties, confusion, restlessness, nervousness, difficulty sleeping, blurry or double vision, ringing in the ears, headache, or convulsions.
Stomach distress also is common with diphenhydramine. Patients may develop a poor appetite, nausea and vomiting, diarrhea or constipation. Patients also may experience low blood pressure, palpitations, a rapid or irregular heart beat, an early onset of menstruation, frequent urination, or difficulty urinating, with urine retained in the bladder.
Diphenhydramine may also cause hives, a rash, sensitivity to the sun, and a dry mouth and nose. Thickened lung secretions are common.
Diphenhydramine's drying effects may be stronger and last longer when taken with an antidepressant called an MAO inhibitor.
Debra Wood, RN
—Something that triggers an allergic reaction
—Agent that blocks or counteracts the action of histamine
Extrapyramidal movement disorders
—Involuntary movements that occur as a side effect of psychiatric medications
—Substance released during allergic reaction