4-27. INTRODUCTION TO ANTIHISTAMINES
In 1933, it was discovered that certain compounds had the property of
counteracting the effects of histamine. These antihistamine compounds directly
antagonize histamine. This action is exerted directly on the peripheral effector cells,
which respond to the histamine. We may think of the antihistamine compound as
putting a protective coating around the susceptible cells and preventing the histamine
from reaching the cells to produce its reaction.
a. Therapeutic Uses. The antihistamines are effective in combating conditions
caused or aggravated by histamine release in the tissues. They do not cure any
condition, but temporarily alleviate symptoms. There is no scientific evidence that
antihistamines either prevent or alleviate the common cold. They provide good
symptomatic treatment for hay fever, drug reactions, serum reactions, some cases of
asthma, and other allergic reactions. Many of the antihistamine compounds are
effective antiemetics, that is, agents used to treat or prevent nausea or vomiting; some
are particularly effective in the prevention and treatment of motion sickness.
b. Untoward Effects. None of the antihistamines is particularly toxic. They
have a high therapeutic index, and truly toxic manifestations are infrequent. Minor side
effects do occur, such as drowsiness, depression, decreased salivation, and nausea.
Drowsiness is the most common side effect. Some have lesser side effects than others
do. Clients under antihistamine medication should be warned by their physician not to
drive or operate heavy machinery. The sedative effect of antihistamines has been put
to use in many sleeping preparations.
4-28. DIPHENHYDRAMINE HYDROCHLORIDE
a. Uses. Diphenhydramine (Benadryl) is highly sedative and is often used as a
sedative for allergic clients. It is also a useful antiemetic in the treatment of motion
sickness and other conditions producing nausea. It may be used to treat parkinsonism.
b. Administration. The usual oral dose is 25 to 50-mg, 3 to 4 times daily. The
usual intramuscular or intravenous dose is 10 to 50-mg.
c. Untoward Effects. Other than sedation, the side effects may include
dizziness, ringing in the ears, incoordination, fatigue, double vision, and nervousness.
In addition, untoward effects involving the digestive tract may occur. These include
loss of appetite, nausea, vomiting, epigastric distress, and constipation or diarrhea.
The incidence of these effects can be reduced by giving the drug with food.
d. Cautions and Contraindications. This drug should not be administered to
people such as truck drivers, aircraft pilots, and others who should stay alert. The
solution of the drug for parenteral use should not be allowed to freeze.