First-Generation (Sedating) Antihistamines

We will address those antihistamines that cause sedation first, which is the older of the two types. Use antihistamines in the management of mild allergic reactions, such as seasonal allergic rhinitis; mild transfusion reactions; and urticaria, which is also referred to as hives and appears as raised, itchy welts or a rash-like eruption on the surface of the skin. You can also use antihistamines during severe allergic reactions of an anaphylactic nature that cause hypotension, acute laryngeal edema, and bronchospasm, and can be life-threatening. You can also treat motion sickness with antihistamines. First-generation antihistamines, which are histamine-1, or H₁ antagonists, are noted for their sedative effects and are often contained in medications that treat insomnia.

Prototype and Other Medications

Diphenhydramine is the prototype medication for sedating antihistamines. Another sedating antihistamine medication is dimenhydrinate.

Expected Pharmacologic Action

Sedating antihistamines bind to H₁ receptors, effectively blocking the release of histamine. This action is what makes diphenhydramine so effective in the treatment of allergic and anaphylactic reactions. Sedating antihistamines are also mild cholinergic blockers, producing actions similar to anticholinergic medications. Subsequently, the anticholinergic effects of dry mouth, constipation, and urinary hesitancy also commonly occur when taking a sedating antihistamine.

Adverse Drug Reactions

Sedating antihistamines produce some adverse drug reactions that are CNS related. The reactions are mild and include drowsiness and dizziness, as well as the previously mentioned anticholinergic effects of dry mouth, constipation, and urinary hesitancy.

Interventions

Because of the sedating effects and potential for dizziness, monitor clients taking sedating antihistamines when ambulating, and advise against operating machinery or driving. If clients experience excessive sedation, recommend to the provider that they switch to a nonsedating antihistamine. To combat the anticholinergic effects of dry mouth, encourage clients to sip water frequently or suck on hard candy. To combat the anticholinergic effect of constipation, recommend the client select menu choices that are high in fiber or obtain a prescription for a fiber supplement or laxative as needed. Monitor clients for urinary retention, especially older men who have urinary hesitancy or enlargement of the prostate gland. Giving the medication with food will help decrease gastrointestinal discomfort. If acute toxicity does develop, administer activated charcoal to neutralize the medication and laxatives to facilitate elimination of the unabsorbed medication from the intestinal tract. Administer acetaminophen for fever and apply ice packs or give sponge baths to further reduce fever. If the client experiences seizures, anticipate giving phenytoin IV.

Administration

As with any type of enteric-coated medication, do not crush or chew the medication. Do not take sedating antihistamines with alcohol or other CNS depressants because of the risk of additional CNS depression. If you give a sedating antihistamine for motion sickness, administer it at least 30 min before the activity that will precipitate motion sickness, and preferably 1 to 2 hr before the activity.

Client Instructions

Instruct clients taking sedating antihistamines to take the medication at or just before bedtime. Also, instruct clients to suck on hard candy (during waking hours) for dry mouth and to increase their fluid and fiber intake for constipation, taking a laxative as needed. If clients have trouble urinating, they need to notify their provider immediately. For clients who experience abdominal discomfort, recommend that they take the medication with food. Provide instructions about how to stop taking the medication and when to seek medical care if manifestations occur.

Safety Alert

Antihistamines, such as diphenhydramine, a common over-the-counter medication, can cause significant sedation and drowsiness when taken. Advise clients taking these medications to avoid driving, operating potentially dangerous machinery, or participating in any activities that require mental alertness.

Contraindications and Precautions

Sedating antihistamines are contraindicated in newborns and children under the age of 2 as well as in women who are breastfeeding. They are also contraindicated in clients who have narrow-angle glaucoma, prostatic hypertrophy, or acute exacerbation of asthma. Use sedating antihistamines with caution in children over the age of 2, in older adults, and in clients who have a history of asthma, urinary retention, open-angle glaucoma, hypertension, and impaired kidney or liver function.

Safety Alert

Clients who have narrow-angle glaucoma need to be told to avoid taking diphenhydramine because it can potentially cause a sudden and painful increase in intraocular pressure that could damage the optic nerve.

Interactions

Sedating antihistamines interact with alcohol and other CNS depressants because they increase the depressant effect of antihistamines. The interaction of these two CNS depressants could cause life-threatening CNS depression.