Antihistamines block histamine release from histamine-1 receptors and are mostly used to treat allergies or cold and flu symptoms, although some first-generation antihistamines (also called sedating antihistamines) may be used for other conditions, such as nausea or motion sickness.
Histamine-1 receptors are located in your airways (breathing tubes), blood vessels, stomach, and esophagus (throat area). Allergens, such as pollen or pet fur can stimulate these receptors, causing the release of histamine, which results in symptoms such as a rash, sneezing, a runny nose, or a narrowing of the airways (also called bronchoconstriction). Histamine-1 receptors are also found in the brain and spinal cord and first-generation antihistamines (sedating antihistamines) can cross through the blood/brain barrier into the brain and act on these receptors, causing sleepiness (sedation), and drowsiness.
Antihistamines can help relieve the symptoms of an allergic reaction, such as:
This makes antihistamines very effective for the treatment of:
Sedating antihistamines (see explanation below) also act in the brain and spinal cord, and on other receptors. This makes some of them also useful for:
Antihistamines can be classified into two main categories:
First-generation (sedating) antihistamines were developed more than seventy years ago and are still in widespread use today. They act on histamine receptors in the brain and spinal cord and in the rest of the body (called the periphery). They also act on muscarinic, alpha-adrenergic, and serotonin receptors. This means that first-generation antihistamines are more likely to cause side effects such as sedation, dry mouth, dizziness, low blood pressure, and a rapid heart-beat. They are also more likely than second-generation antihistamines to impair a person’s ability to drive or operate machinery. Interactions with other drugs are more common with first-generation antihistamines compared with second-generation antihistamines.
Second generation (non-sedating) antihistamines were developed in the 1980s and are much less sedating than first-generation antihistamines. They act on histamine-1 receptors in the periphery and are unlikely to penetrate the brain, so are less likely to cause side effects or interact with drugs. Most second-generation antihistamines do not cause drowsiness, although some (such as cetirizine and fexofenadine), may be more likely to do so at higher dosages.
| Generic name | Brand name examples | Duration of action |
|---|---|---|
| brompheniramine | Generic | 12h |
| carbinoxamine maleate | Generic | 12h |
| chlorpheniramine | Aller-Chlor | 24h |
| clemastine | Generic | 12h |
| diphenhydramine | Benadryl | 12h |
| hydroxyzine | Vistaril | 24h |
| triprolidine | Discontinued | 12h |
| Generic name | Brand name | Duration of action |
|---|---|---|
| azelastine nasal spray | Astelin | 12h |
| cetirizine | Zyrtec Allergy, Children’s Zyrtec | 24h+ |
| desloratadine | Clarinex, Clarinex Reditabs | 24h+ |
| fexofenadine | Allegra Allergy, Children's Allegra Allergy | 12-24h |
| levocetirizine | Xyzal | 24h+ |
| loratadine | Alavert, Claritin | 24h |
| olopatadine nasal spray | Patanase | 12h |
Side effects of first-generation (sedating) antihistamines may include:
Side effects of second-generation (non-sedating) antihistamines may include:
Data Source:drugs.com
Benztropine is used along with other medications to treat the symptoms of Parkinson's disease (PD; a disorder of the nervous system that causes difficulties with movement, muscle control, and balance) and tremors caused by other medical problems or medications. Benztropine is in a class of medications called anticholinergics. It works by blocking a certain natural substance (acetylcholine) to help to decrease symptoms in people with Parkinson's disease.
Data Source:medlineplus.gov
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The combination of doxylamine and pyridoxine is used to treat nausea and vomiting in pregnant women whose symptoms have not improved after changing their diet or using other non-medicine treatments. Doxylamine is in a class of medications called antihistamines. It works by blocking the action of certain natural substances in the body that may contribute to nausea and vomiting. Pyridoxine (vitamin B6) is a vitamin. It is given because a lack of pyridoxine in the body may also be a factor in causing nausea and vomiting during pregnancy.
Data Source:medlineplus.gov
Hydroxyzine is used in adults and children to relieve itching caused by allergic skin reactions. It is also used alone or with other medications in adults and children to relieve anxiety and tension. Hydroxyzine is also used along with other medications in adults and children as a sedative before and after general anesthesia for surgery. Hydroxyzine is in a class of medications called antihistamines. It works by blocking the action of histamine a substance in the body that causes allergic symptoms. It also works by decreasing activity in the brain.
Data Source:medlineplus.gov
Hydroxyzine is used in adults and children to relieve itching caused by allergic skin reactions. It is also used alone or with other medications in adults and children to relieve anxiety and tension. Hydroxyzine is also used along with other medications in adults and children as a sedative before and after general anesthesia for surgery. Hydroxyzine is in a class of medications called antihistamines. It works by blocking the action of histamine a substance in the body that causes allergic symptoms. It also works by decreasing activity in the brain.
Data Source:medlineplus.gov
Hydroxyzine is used in adults and children to relieve itching caused by allergic skin reactions. It is also used alone or with other medications in adults and children to relieve anxiety and tension. Hydroxyzine is also used along with other medications in adults and children as a sedative before and after general anesthesia for surgery. Hydroxyzine is in a class of medications called antihistamines. It works by blocking the action of histamine a substance in the body that causes allergic symptoms. It also works by decreasing activity in the brain.
Data Source:medlineplus.gov
