Antihistamines Flashcards
What is the MOA for antihistamines?
Block action of histamine at receptor sites
Histamine is released from ______ and causes _______
Mast cells
Vasodilation, Increased capillary permeability, and increased GI and pulmonary secretions
What are the possible routes of administration for antihistamines?
Oral Nasal spray Eye drops Topical IM IV
Clinical indications for antihistamines
Allergic conditions (rhinitis, dermatitis, conjunctivitis)
Hypersensitivity reactions to drugs
Anaphylaxis
Urticaria (acute or chronic)
Motion sickness
Vertigo
Insomnia
1st gen antihistamines block histamine at the ________
H1 receptor
Why do 1st gen antihistamines have more CNS side effects?
Because they readily cross the BBB
Name the 1st gen oral antihistamines
Diphenhydramine (Benadryl)
Chlorpheniramine (Chlor-trimeton)
Dimenhydrinate (Dramamine)
Promethazine (Phenergan)
Meclizine (Antivert)
Hydroxyzine (Atarax, Vistaril)
Which H1 blocker is best for vertigo?
Meclizine (Antivert)
Which H1 blocker causes the most extreme sedation?
Hydroxyzine (Atarax)
2nd gen H1 blockers are known to be …
Less sedating than 1st gen (b/c don’t cross the BBB as much)
What is the one 2nd gen H1 blocker that can be sedating
Cetirizine (Zyrtec)
What are the three 2nd gen H1 blockers?
Loratadine (Claritin)
Cetirizine (Zyrtec)
Fexofenadine (Allegra)
Name the two H2 blockers we discussed
Ranitidine (Zantac)
Famotidine (Pepcid)
What are some examples of nasal spray antihistamines?
Azelastine (Astelin, Astepro)
Azelastine + Fluticasone (Dymista)
What are some examples of antihistamine eye drops?
Azelastine (Optivar)
Ketotifen (Zatidor) AH + MCS
Naphazoline (Naphcon A)
Olopatadine (Patanol) AH + MCS
Can you use 2nd gen H1 blockers for motion sickness or insomnia?
NOPE
You need the anticholinergic properties of 1st gens, that cross the BBB
What are the side effects of 1st gen H1 blockers?
ANTICHOLINERGIC
Red as a Beet Dry as a Bone Blind as a Bat Mad as a Hatter Hot as a Hare
Which antihistamines are approved by use by pilots after an adequate trial without SE
Loratadine (Claritin)
Fexofenadine (Allegra)
Others (including Zyrtec) are not allowed
What does the FAA require for use of any non-approved antihistamine?
Pilot must ground themselves for at least 5x the drug’s half-life (for Benadryl, that would be 60 hours)
Other occupations that should avoid first gen H1 blockers other than pilots
Heavy machinery operator
Construction workers or electricians working at heights
Mass transit (ie bus driver)
Healthcare providers working 24 hour shifts or night shifts
PA students who cannot fall asleep during class
What would be your recommendations for an elderly patient with BPH who’s hay fever is not responsive to his current regimen of Benadryl and Sudafed?
Stop both drugs - you want to avoid decongestants and 1st gen h1 blockers as they may trigger urinary retention
Use nasal steroid first line instead
What drugs are contraindicated in patients with glaucoma?
Decongestants, antihistamines, and steroids (including nasal steroids) - all may increase IOP
How do you approach treating hay fever in a patient with glaucoma?
Consult patient’s ophthalmologist to determine plan
May require closer monitoring of IOP if requiring long term allergy tx
Consider immunotherapy (allergy shots)
For patients with glaucoma or BPH, avoid using…
Phenergen for gastroenteritis (use zofran instead)
Antivert for labyrinthitis
H1 blockers for motion sickness or insomnia
Scabies is infestation of the skin by…
The mite Sarcoptes scabiei
—> intense nighttime itching and burrow lesions
DOC for scabies
Permethrin cream 5% from neck to soles of feet for 8-12 hours
Consider Atarax (hydroxyzine) or Benadryl for sedation
Chronic urticaria is sometimes linked to…
Autoimmune causes
Chronic urticaria will typically respond well to …
Non-sedating antihistamines (ie Claritin, Allegra)
How do you treat poison ivy?
Steroid cream (ie Triamcinolone or Kenalog) - remember, no high potency over face
Non-sedating oral antihistamine (ie Zyrtec)
Might need to add oral steroid taper if not improving
Warn patient to watch for signs of infection
Your dumbass patient with a PCN allergy took his buddy’s augmentin for some unknown reason. How do you treat the rash he developed?
Stop the augmentin
Give h1 blocker (Zyrtec ok)
Consider IM steroids and/or IM antihistamine - don’t let them drive themselves home after this
Consider tapering course of oral steroids based on severity
What drug is not part of the first line treatment for urticaria but can be a useful adjunct if patient not responding to treatment?
H2 blockers (Ranitidine or Famotidine)
How do you treat anaphylaxis?
EPINEPHRINE
Benadryl
IM/IV glucocorticoid (ie Dexamethasone)
H2 blocker (Famotidine, Ramitidine)
Severe potentially life-threatening allergic reaction to food, meds, latex, insect stings etc
Anaphylaxis
Can affect >1 body part at the same time • Skin and mucosa - 90% • Respiratory - 70% • GI - 45% • Cardiac (hypotension, syncope, tachycardia) - 45%
