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.
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.
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 theraputic 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.
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.
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 accurate results.
Elderly people are more sensitive to the sedating effects of diphenhydramine. The drug may also cause dizziness and lower blood pressure. Older people should slowly change position from sitting or lying to standing while taking this medication to prevent dizziness and fainting.
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, or 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.
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.
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.
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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, x2600 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., M.D., Dr.P.H.