The first generation antihistamines are the older group of antihistamines and are often used as ‘sleeping aids’ because of its strong sedative actions.
Although the sedative effect of antihistamines are sometimes desired, like in a patient with allergic rhinitis who is having difficulty sleeping, generally the use of these drugs should be limited in a person who is driving or operating heavy machinery.
If a patient has a deficiency or is completely lacking this enzyme an adequate analgesic effect will not be obtained.
Estimates indicate that up to 7% of the caucasian population may have this deficiency.
Digestive disturbances—loss of appetite, nausea, a bloating sensation, constipation, and diarrhea—are particularly common adverse drug reactions, because most drugs are taken by mouth and pass through the digestive tract. In older people (see Aging and Drugs), the brain is commonly affected, often resulting in drowsiness and confusion.
Some adverse drug reactions are identified when a drug is being tested before it is submitted to the Food and Drug Administration (FDA) for approval.
Doxepin is an antihistamine with additional antidepressant actions.