Antihistamines work on the H1 receptors by blocking them so they can’t get stimulated. So, not surprisingly, the effects of antihistamines are the opposite of what histamine normally does.
The same way Cromolyn is useless if a person is already having an allergic reaction, antihistamines cannot work acutely either. It will take 3 days for it to work if histamines were already released because it takes time for the histamine to be pulled off the receptor.
Antihistamines are found in cold medications. Even though a cold is not an allergic reaction, they are useful in the treatment of the symptoms because of their anticholinergic effects (the drying effects due to decreased mucous production). So it’s not the antihistaminic effect that’s working but the anticholinergic.
- Blocks H1 receptors so that histamines cannot bind and stimulate
- Onset of H1 blockade can be delayed 3 days if histamines have already been released
- Other properties
- anticholinergic (sedation; drying)
Ineffective for:
- anaphylactic shock
- acute bronchial asthma
- common cold
- but we use it for the anticholinergic properties
Antihistamine Pharmacology
Antihistamines have basically the opposite effects of we mentioned in regards to histamine.
Vascular: Permeability would be decreased and there would not be any vasodilation.
Nervous System: The itch that normally takes place with histamine won’t be present.
Smooth muscle: It antagonizes bronchoconstriction. However, bronchoconstriction that takes place with an allergic or asthmatic reaction wont be treated with an antihistamine because if the effect is already there it would take at least 3 days for it to be reversed with an antihistamine. Just remember it won’t work acutely.
Side effects: Sedation but some more-so than others. For example Claritin claims that it’s the non-drowsy medication but it still has some effect. If you look at this drug list, the one most sedating is Benadryl. And you know that because if you look at OTC sleep medications, there is diphenhydramine in it. Also keep in mind that antihistamines have anticholinergic properties.
Recap
- Vascular
- Blocks permeability and vasodilation
- Nervous System
- Blocks itch
- Smooth Muscle
- Antagonizes bronchoconstriction
- Not acute therapy
- Side Effects
- Sedation but some moreso than others.
- Anticholinergic
Therapeutic Use
- Symptomatic relief of allergies
- Exudative allergies like a runny nose
- Noninflammatory skin conditions (allergic dermatoses) such as hives (urticaria)
- Pruritis (itchy skin)
Antihistamine List
- Diphenhydramine (Benadryl): highly sedating
- Cetirizine (Zyrtec): highly sedating
- Brompheniramine Maleate (Dimetane): less sedating than above 2
- Chlorpheniramine Maleate (Chlor-Trimeton): less sedating than above 2
- Fexofenadine (Allegra): long acting, low incidence of drowsiness
- Loratadine (Claritin): long acting, low incidence of drowsiness
- Cyproheptadine (Periactin)
- Hydroxyzine (Atarax, Vistaril)
- Desloratadine (Clarinex)
Now let’s move on from antihistamines and look at other classes of medications use for respiratory therapy: Bronchodilator Drugs / Treatment for Chronic Obstructive Pulmonary Disease