Allergies ResourcePharm Qs Flashcards
What is the primary mechanism of action of antihistamines?
Antihistamines bind to histamine H1 receptors and block the effects of histamine.
List at least five indications for the use of antihistamines.
Allergic rhinitis, vasomotor rhinitis, allergic conjunctivitis, urticaria, angioedema, insect bites and stings, nausea and vomiting, motion sickness, insomnia associated with urticaria and pruritus, and adjuvant therapy in the treatment of anaphylaxis.
How are antihistamines generally administered?
Antihistamines are typically administered orally, but some can be given intravenously, intramuscularly, or topically (e.g., as nasal sprays or eye drops).
What are the two classifications of antihistamines?
First-generation (sedating) and second-generation (non-sedating) antihistamines.
Why do sedating antihistamines cause sedation?
Sedating antihistamines are highly lipid-soluble and readily cross the blood-brain barrier, leading to sedation.
What are some anticholinergic side effects of sedating antihistamines?
Dry mouth, blurred vision, dizziness, constipation, tachycardia/palpitations, sedation, and urinary retention.
Why should sedating antihistamines be used with caution in older adults?
Older adults are more sensitive to anticholinergic activity, which can lead to a higher risk of falls, fractures, and cognitive impairment.
Name at least three examples of sedating antihistamines.
Alimemazine, chlorphenamine, cyproheptadine
Hydroxyzine
Ketotifen
Promethazine hydrochloride
Promethazine teoclate
Cinnarizine
Name at least three examples of non-sedating antihistamines.
Acrivastine, bilastine, cetirizine, Desloratadine
Fexofenadine
Levocetirizine
Loratadine
Mizolastine
Rupatadine
What is the preferred choice of antihistamine for most patients and why?
Non-sedating antihistamines are preferred due to the lower risk of adverse effects compared to sedating antihistamines.
What should patients be aware of when using non-sedating antihistamines?
Sedation is rare, but some patients may experience it, which can affect tasks like operating machinery or driving.
Why are sedating antihistamines sometimes used in managing eczema?
Sedating antihistamines can help manage disturbed sleep due to pruritus (itching).
Why might sedating activity be helpful in eg eczema patients?
Helpful as sleep may be disturbed due to pruritus (severe itching)
Why do sedating antihistamines have significant anticholinergic activity?
Sedating antihistamines also act on the muscarinic receptors as antagonists
Who should sedating antihistamines be used in caution with?
patients with prostatic hypertrophy, urinary retention and angle-closure glaucoma.
(One of the links on the page) Are there situations where first-generation antihistamines are better than newer ones, especially when it comes to things like itching from conditions like atopic dermatitis?
Newer antihistamines are generally preferred because they last longer and have fewer side effects. They are also less likely to cause harmful drug interactions or toxicity, which is a concern with first-generation antihistamines. First-gen antihistamines can make you drowsy and impair your focus, which can affect kids’ learning or adults’ ability to drive. However, in cases of severe itching, like in atopic dermatitis, a first-generation antihistamine like hydroxyzine might actually help, since its sedating effect could be beneficial at night. For pregnancy, cetirizine and loratadine are preferred, while first-generation antihistamines should be avoided. Overall, first-generation antihistamines are usually not recommended.
How does the duration of action differ between first-generation and second-generation antihistamines?
First-generation antihistamines have a shorter duration of action, whereas second-generation antihistamines have a longer duration of action.
How does selectivity for histamine H1 receptors differ between first-generation and second-generation antihistamines?
First-generation antihistamines act on both peripheral histamine H1 receptors and central histamine H1 receptors in the central nervous system, whereas second-generation antihistamines have high affinity and selectivity for peripheral histamine H1 receptors, resulting in the least possible central nervous system side effects.
How does binding affinity for muscarinic and alpha-adrenergic receptors compare between first-generation and second-generation antihistamines?
First-generation antihistamines have a higher binding affinity for muscarinic and alpha-adrenergic receptors, whereas second-generation antihistamines have a lower binding affinity.
How do the side effect profiles of first-generation and second-generation antihistamines compare?
First-generation antihistamines cause more side effects, including drowsiness, fatigue, cognitive impairment, dizziness, and dry mouth, whereas second-generation antihistamines cause fewer side effects and are less likely to cross the blood-brain barrier, resulting in minimal drowsiness or no sedation.
How do first-generation and second-generation antihistamines differ in terms of clinically relevant drug interactions?
First-generation antihistamines have possible clinically relevant drug interactions, whereas second-generation antihistamines are unlikely to have such interactions.
How does the abuse potential compare between first-generation and second-generation antihistamines?
First-generation antihistamines are more likely to be abused, whereas second-generation antihistamines are less frequently abused.
Why are second-generation antihistamines recommended over first-generation antihistamines for older people?
The elimination half-life and duration of action of first-generation antihistamines are prolonged in older people, and their sensitivity to sedative and anticholinergic effects (such as drowsiness, confusion, and agitation) is amplified, increasing the risk of adverse effects. Second-generation antihistamines are recommended over first-generation antihistamines for this population.
Patient Counselling/Monitoring
Monitor patient for anticholinergic side effects and other adverse effects.
Give advice on how to manage the following anticholinergic side effects:
Antihistamines are most effective when taken before exposure to the allergen
-Dry mouth: suck on a sugar-free lozenge or sweet or take sips of water and
monitor
- Constipation: increase fibre and water intake and monitor
Assess whether the antihistamine is providing relief from symptoms
associated from the allergy or do they need to be switched to another
antihistamine
Provide advice on how to reduce exposure to allergens
