Diphenhydramine
Diphenhydramine
Definition
Diphenhydramine is an antihistamine used in psychiatric medicine to treat phenothiazine drug-induced abnormal muscle movement. It is also used in general medicine to treat allergies, allergic reactions, motion sickness, insomnia , cough, and nausea. When diphenhydramine is used for allergy-related symptoms, it is sold in the United States as an over-the-counter medication Benadryl. For use in the treatment of the tremors caused by phenothiazines, diphenhydramine is prescribed in the generic form.
Purpose
Some drugs called phenothiazines are used to treat psychotic disorder such as schizophrenia. As a side effect, these drugs may cause tremors and abnormal involuntary movements of the muscles, referred to as extrapyramidal neurologic movement disorders. Diphenhydramine is used to control these symptoms. Other uses of the drug include the treatment of nausea, vomiting, and itching. Diphenhydramine is used to help limit allergic reactions to transfused blood products. It can induce sleep. It is sometimes used to treat the stiffness and tremor of Parkinson’s disease. In liquid form, it may relieve minor throat irritation.
Description
Diphenhydramine is an antihistamine that is readily distributed throughout the body. It is easily absorbed when taken by mouth. Maximal action occurs approximately one hour after swallowing the drug. The effects continue for four to six hours. Diphenhydramine acts on cells in the brain. It seems to compete with the chemical histamine for specific receptor sites on cells in the brain and central nervous system. This means that it achieves its therapeutic effect by taking the place of the neurotransmitter histamine on these cells. Diphenhydramine is a useful medication for individuals with mild parkinsonism when it is used in combination with centrally acting anticholinergic drugs.
Recommended dosage
The dosage of diphenhydramine must be adjusted according to the needs of individuals and their responses. Adults are generally given 25-50 mg orally, three to four times daily. Diphenhydramine may be administered through a vein or injected deep within a muscle. The usual dosage is 10-50 mg per injection, although some people may require 100 mg. The total daily dosage should not exceed 400 mg. People who forget to take a dose of this drug should skip the dose and take the next one at the regularly scheduled time. They should not double up subsequent doses if one is missed.
People should not take diphenhydramine if they are taking other preparations that contain antihistamines unless specifically directed to do so by a physician.
Precautions
People with peptic ulcer disease, bowel obstructions, an enlarged prostate, angle closure glaucoma, or difficulty urinating due to a blockage in the bladder should not use diphenhydramine without close physician supervision and monitoring. People with asthma, heart disease, high blood pressure, or an overactive thyroid should use this drug with caution. Before taking diphenhydramine, people with these conditions should discuss the risks and benefits of this drug with their doctor. Individuals should not take diphenhydramine for several days before an allergy test, as it will interfere with the results.
Elderly people are more sensitive to the sedating effects of diphenhydramine. The drug may also lower blood pressure and cause dizziness. Older people should slowly change position from sitting or lying to standing while taking this medication to prevent dizziness and fainting.
Side effects
Drowsiness commonly occurs after taking diphenhydramine. This effect may be more pronounced if alcohol or any other central nervous system depressant, such as a tranquilizer or a particular medication for pain, is also taken. People taking the drug should not drive, operate machinery, or perform hazardous tasks requiring mental alertness until the effects of the medication have worn off. In some people, diphenhydramine also may cause dizziness, difficulties with coordination, confusion, restlessness, nervousness, difficulty sleeping, blurry or double vision, ringing in the ears, headache, or convulsions.
KEY TERMS
Anticholinergic —Related to the ability of a drug to block the nervous system chemical acetylcholine. When acetylcholine is blocked, patients often experience dry mouth and skin, increased heart rate, blurred vision, and difficulty in urinating. In severe cases, blocking acetylcholine may cloud thinking and cause delirium.
Antihistamine —A medication used to alleviate allergy or cold symptoms such as runny nose, itching, hives, watering eyes, or sneezing.
Extrapyramidal movement disorders —Involuntary movements that occur as a side effect of some psychiatric medications.
Histamine —Substance released during allergic reactions.
Hypokinesia —A condition of abnormally diminished motor activity.
Parkinson’s disease —A disease of the nervous system most common in people over 60, characterized by a shuffling gait, trembling of the fingers and hands, and muscle stiffness.
Parkinsonism —A condition caused by the destruction of the brain cells that produce dopamine (a neurotransmitter); characterized by tremors of the fingers and hands, a shuffling gait, and muscular rigidity.
Phenothiazine —A class of drugs widely used in the treatment of psychosis.
Stomach distress is a relatively common side effect of diphenhydramine. Some people may develop poor appetites, nausea, vomiting, diarrhea, or constipation. Individuals also may experience low blood pressure, palpitations, rapid or irregular heartbeats, frequent urination, or difficulty urinating. Urine may be retained in the bladder. Other side effects of diphenhydramine are associated with persons in age groups that are unlikely to use the drug.
Diphenhydramine may also cause hives, a rash, sensitivity to the sun, and a dry mouth and nose. Thickened lung secretions are common among older persons.
Interactions
Alcohol, pain medications, sleeping pills, tranquilizers, and antidepressants may make the drowsiness associated with diphenhydramine more severe.
Diphenhydramine should not be used by persons taking hay-fever medicines, sedatives , narcotics, anesthetics, barbiturates , or muscle relaxants.
Resources
BOOKS
Foreman, John C. and Torben Johansen. Textbook of Receptor Pharmacology, 2nd ed. Boca Raton, FL: CRC Press, 2002.
Page, Clive P., and Michael Murphy. Integrated Pharmacology. St. Louis: Mosby-Year Book, 2002.
Preston, John D., John H. O’Neal, and Mary C. Talaga. Handbook of Clinical Psychopharmacology for Therapists, 4th ed. Oakland, CA: New Harbinger Publications, 2004.
PERIODICALS
Grillon, Christian, and others. “The Benzodiazepine Alprazolam Dissociates Contextual Fear from Cued Fear in Humans as Assessed by Fear-Potentiated Startle.” Biological Psychiatry 60.7 (Oct. 2006): 760–66.
Khoromi, Suzan, and others. “Topiramate in Chronic Lumbar Radicular Pain.” Journal of Pain 6.12 (Dec. 2005): 829–36.
Scaife, J. C., and others. “Sensitivity of Late-Latency Auditory and Somatosensory Evoked Potentials to Threat of Electric Shock and the Sedative Drugs Diazepam and Diphenhydramine in Human Volunteers.” Journal of Psychopharmacology 20.4 (July 2006): 485–95.
Turner, Claire, Alison D. F. Handford, and Anthony N. Nicholson. “Sedation and Memory: Studies with a Histamine H-1 Receptor Antagonist.” Journal of Psychopharmacology 20.4 (July 2006): 506–17.
ORGANIZATIONS
American Academy of Family Physicians. 11400 Tomahawk Creek Parkway, Leawood, KS 66211-2672. Telephone: (913) 906-6000. Web site: <http://www.aafp.org>.
American College of Physicians. 190 N Independence Mall West, Philadelphia, PA 19106-1572. Telephone: (800) 523-1546, ext. 2600 or (215) 351-2600. Web site: <http://www.acponline.org>.
American Medical Association. 515 N. State Street, Chicago, IL 60610. Telephone: (312) 464-5000. Web site: <http://www.ama-assn.org>.
American Parkinson Disease Association. Inc. 1250 Hylan Boulevard Suite 4B, Staten Island, NY 10305-1946. Telephone: (800) 223-2732 or (718) 981-8001. Fax: (718) 981-4399. Web site: <http://www.apdaparkinson.com>.
American Psychiatric Association. 1400 K Street NW, Washington, DC 20005. Telephone: (888) 357-7924. Fax (202) 682-6850.
American Society for Clinical Pharmacology and Therapeutics. 528 North Washington Street, Alexandria, VA 22314. Telephone: (703) 836-6981. Fax: 703-836-5223.
American Society for Pharmacology and Experimental Therapeutics. 9650 Rockville Pike, Bethesda, MD 20814-3995. Telephone: (301) 530-7060.
Parkinson’s Disease Foundation. 710 West 168th Street, New York, NY 10032-9982. Telephone: (800) 457-6676 or (212) 923-4700. Fax: (212) 923-4778. Web site: <http://www.pdf.org>.
L. Fleming Fallon, Jr, MD, Dr.P.H.
Ruth A. Wienclaw, PhD