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10.01 Histamine 1 Receptor Blockers

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  1. Histamine 1 Receptor Blockers
    1. Mechanism of action
      1. Patho background
        1. Histamines are part of the bodies inflammatory reaction to allergens
          1. Histamine binds to histamine receptors causing-
            1. Itching
            2. Increased mucus secretion
            3. Flushing
            4. Headache
            5. Tachycardia
            6. Bronchoconstriction
            7. Increased vascular permeability
            8. Edema
            9. Hypotension
      2. Histamine 1 Blockers blocks the receptor so the inflammatory reaction doesn’t happen.
    2. Indication
      1. Allergic rhinitis
      2. Urticaria
      3. Pruritis
      4. Anaphylactic reaction
        1. Primary treatment is Epinephrine to treat airway edema, but Histamine blockers help stop allergic reaction

Nursing Points


  1. Examples of Histamine 1 Blockers
    1. First generation
      1. Has more side effects than other generations
      2. Diphenhydramine
      3. Chlorpheniramine
      4. Promethazine
    2. Second generation
      1. Has fewer side effects than first generation
      2. Cetrizine
      3. Loratadine
    3. Third Generation
      1. The least side effects
      2. Fexofenadine


  1. Assess for side effects
    1. First generation
      1. Sedation/drowsiness
      2. Respiratory depression
      3. Decreased ability to focus
      4. Dry mouth
      5. Urinary retention
      6. N/V
      7. Hypotension
      8. Paradoxical Excitation- when medication suddenly stopped
        1. Insomnia
        2. Restlessness
        3. Anxiety
        4. Dystonia
        5. Akathisia
        6. Parkinsonism
        7. Tardive dyskenisa
      9. Agranulocytosis (severely lowered WBC)
      10. Impaired immune system
    2. Second & Third generation
      1. Little to no affect on sedation
      2. Very few side effects to note

Therapeutic Management

  1. Monitor for sedation and respiratory depression
  2. Medications are available over the counter- patient education essential

Nursing Concepts

  1. Pharmacology
    1. Histamine 1 Blockers are commonly prescribed to treat allergic reactions.

Patient Education

  1. Patients should be instructed to not operate heavy machinery when taking First Generation Histamine 1 Blockers because of possible sedation.
  2. Patients should be instructed not to stop First Generation Histamine 1 Blockers suddenly because of the risk of Paradoxical Excitation.

Study Tools

Video Transcript

Video Transcript

So, we gonna talk about histamine receptor blockers. Now, there are two main categories in these medications; Histamine 1 Receptors blockers and Histamine 2 Receptors blockers. In this video, we gotta take a look at about the Histamine 1 Receptor blockers and what are the mechanism of action, side effects, indication and the names of the medication. And we’ll take a look at this Histamine 2 Receptors blockers in a different video, okay?

So, let’s take a look at the mechanism of action of this medication. So, we have a histamine which is inflammatory mediator in our body. What it means, like whenever any kind of allergen enters our body, there’s inflammatory reaction happens and the main mediator is basophil and mast cells. They releases this histamine in our body, I mean in our blood. And, this histamine goes and binds to their histamine receptors, histamine 1 receptors. When it binds to histamine 1 receptors, it causes all these kinds of reaction, I mean, all these kinds of signs and symptoms. Signs and symptoms right here like itching, increased mucus secretion, that’s why you have a runny nose, nasal congestion, flushing, headache, tachycardia, bronchoconstriction, increased in vascular permeability, that’s why they will have edema and all kinds of stuff right here, and it will cause a hypotension. So, those are basically the normal inflammatory signs and symptoms you’ll see if you have allergy to pollen and if you get in contact with a pollen, then your body reacts this way. Now, what this medication does, it actually blocks the histamine 1 receptors. So, right here, it blocks this histamine 1 receptors. So when it blocks the histamine 1 receptor, the histamine won’t be able to bind to those receptors and produce any of these effects. So, that’s the basic mechanism of action is to bind to histamine 1 receptors and block them. So, the histamine cannot produce these effects which is itching, nasal congestion, headache, tachycardia, bronchoconstriction, hypotension and edema.

So, now we learn, this is basically, when it happens like when you are getting contact with alllergen, this kind of reaction happens. Now, this medication uses for allergic rhinitis, obviously, urticartia, pruritis. These medications also used for anaphylactic reaction, but, however, you really need to be careful in this statement in conjunction with epinephrine. So, you can give a patient this medications in order to relieve those symptoms, however, you have to give epinephrine at the same time if they are going in anaphylactic reaction. Now, the main difference between, like people always get confuse, what’s the main difference between allergic reaction and anaphylactic reaction? So you have allergic and anaphylatic reaction. In allergic reaction, there won’t be any involvement of respiratory. Like, it can cause bronchoconstriction but not at the extent that will prevent patient breathing. While in anaphylactic reaction, they will have like swelling around their airway, swelling around airway, and that will make them hard to breath. So, basically, that’s the main difference in anaphylactic reaction. Patient won’t be able to breathe normally like they’ll be really having the hard time. While the allergic reaction they may have a bronchoconstriction, however, they’ll be breathing normally. I mean, not normally, but they’ll be breathing, while not in the anaphylactic. That’s why you need to give epinephrine in order to reverse that swelling around the airway and cause bronchovasodilation in anaphylactic reaction. We can give this medication in conjunction with epinephrine as well. Now, when you have like allergic reaction, it causes the sneezing, runny nose, itching of the eyes. So, this medication is also helpful to prevent those signs and symptoms as well in allergic reaction. And this medication actually causes sometimes significant sedation. So, this is kind of off label use. People use this medication for sedation. Like the main widely over-the-counter medication used for sedation from this category is Benadryl.

Alright. Now, in this video, we gonna talk about what are the examples of this medication before we go into the side effects. Because, I’ll tell you why we did this one. So, this is the first generation medication Histamine 1, second and third. These are really kinda hard to remember the names. There’s no like a, no easy way to remember except repeating everyday in order to remember better. So, some of the main ones are Diphenhydramine, Chlorpheniramine, Promethazine, Dimenhydrinate. Those are the widely used ones. In second Generation, Cetirizine, Loratadine. The third one is Fexofenadine. Those are the widely used medication in the United States. Now, basically, the first generation medication are older drugs in this category, while this second one are a little bit newer and third one are newer than second generation. And, third generation has actually less side effects than the first generation and second generation. And, second generation has less side effects than first generation, like that. So, that’s why, I just wanted to point it out before we go to the side effects. Like, one of the main side effects of this category is sedation. Third generation, actually, doesn’t cause that much sedation at an extent that will impair your ability to work. While the first generation can cause really significant sedation. They’ll make you drowsy, you can’t really focus. That’s why when a patient takes this medication over-the-counter, you want to really educate, like do not drive right after you take this medication. Allow at least 8 hours of sleep after you take this medication because you can’t really take this medication and just have like 4 hours sleep and then go to work or do some cognitive abilities. ‘Cause this is gonna impair your ability to do those activities significantly.

Let’s take a look at the side effects. First of all, dry mouth, headache, dizziness, it can cause urinary retention, cause nausea, vomiting, obviously the sedation, and hypotension. And this is the one, sedation, as we talked in the previous slide. The first generation histamine 1 receptor blockers will cause a significant sedation compared to the third one, third generation. Now this one, there’s other side effects in a separate box. These are the, like really, very lethal side effects like Paradoxical Excitation. So, if someone suddenly stops this medication, then they can have Paradoxical Excitation like since this medication causes sedation, it will cause insomnia, restless, anxiety, and so forth. Like it can cause really bad paradoxical excitation. It can cause hypersensitivity reaction. Now, this medication can also cause the extrapyramidal symptoms, And what are those? Dystonia, akathisia, parkinsonism and tardive dyskinesia. So, those are the extrapyramidal symptoms and we’ll go in these symptoms in detail in anti-psychotic medications. But those are really the bad side effects of medication. It can have also, this medication can cause Agranulocytosis and it can significantly decrease your WBCs, it can impair your immune system as well. And this medication, since it causes sedation, it definitely can cause respiratory depression as well. So, you have to be really careful and need to do a really good education if patient, one of the patient is taking this over-the-counter medication at home to be careful about the side effects.

I think that was it about the Histamine 1 receptor blockers. Thanks for watching. If you have any questions, just let us know. Thank you.