Antihistamines

Introduction

In a community pharmacy setting, almost on a daily basis, we recommend oral antihistamine to our customers, which could be for prevention and treatment of symptoms in allergic rhinitis, allergic conjunctivitis, urticaria, or a variety of other allergic diseases.

1st and 2nd Generation

As an overview, antihistamines can be classified into 1st and 2nd generation. This dichotomy was introduced in 1983 to indicate a major pharmacological difference between terfenadine and astemizole.

• Being less lipid soluble, the second generation antihistamines is thus believed to be less readily penetrated the blood brain barrier, and hence does not cause drowsiness.

However, second generation antihistamines are still able to penetrate the blood brain barrier and may cause drowsiness (thought it rarely occurs).

• In fact, there are textbooks using the terminology of "less sedating antihistamines" to describe the second generation antihistamines, instead of "non-sedating antihistamines".
• Based on a study in year 2000, the odds ratios (adjusted for age and sex) for the incidence of sedation were 3.53 for cetirizine when compared to loratadine.

In terms of efficacy, both first and second generation antihistamines have similar efficacy.

• Individual response to antihistamine varies widely; it may be necessary to try a number to see which is best tolerated and most effective.
• However, the sedative effect will certainly play a key role in recommending appropriate medicine to your patients.

NOTE:

• Levocetirizine is actually the active R-enantiomer of cetirizine.
• Desloratadine, on the other hand, is the active metabolite of loratadine.

Indications

Theoretically, antihistamines could reduce some of the symptoms of a cold: runny nose and sneezing. These effects are probably due to the anticholinergic action of antihistamines.

• The older drugs (e.g. chlorpheniramine, promethazine) have more pronounced effects than do the less sedating antihistamines (e.g., loratadine, cetirizine, desloratadine, levocetirizine, fexofenadine).
• They are not effective at reducing nasal congestions.
• Evidence indicates that antihistamines alone or in combination with decongestants, analgesics and cough suppressants may only offer limited benefits in the common cold for adults.

Antihistamines are also first line treatment of chronic urticaria, but many patients do not get satisfactory relief with recommended doses.

• To address this issue, European guidelines recommend to increase the dosage of non-sedating antihistamine up to four-fold. Second-line therapies should then be added to the antihistamine treatment if patients do not respond to a four-fold increase in dosage of nonsedating antihistamines.
• In other words, in practice, you may see a patient to be prescribed with cetirizine 20 mg twice daily for management of chronic spontaneous urticaria.

Pseudoephedrine

I sometimes found it frustrating to come across prescriptions that instruct patients to take Clarinase (loratadine and pseudoephedrine) twice a day for a month.

• While Clarinase can work wonders for nasal congestion and is readily available at commnity pharmacies, the question arises: is the long-term prescribing of this medication justified?

Unlike topical formulation, oral sympathomimetics can have systemic effects.

• To illustrate, sympathomimetics can cause stimulation of the heart and an increase in blood pressure and may affect diabetic control because they can increase blood glucose levels. They should be used with caution (as per current British National Formulary (BNF) warnings) in people with diabetes, those with heart disease or hypertension and those with hyperthyroidism. The hearts of hyperthyroid patients are more vulnerable to irregularity, so stimulation of the heart is particularly undesirable.

Additionally, we must not overlook the potential risk of pseudoephedrine abuse associated with this medication.

External Links

• H1 Antihistamines: Current Status and Future Directions, 2008
• Antihistamines for the common cold, 2015
• What treatments are effective for common cold in adults and children?, 2018
• EAACI/GA²LEN/EDF/WAO guideline: management of urticaria, 2009
• The effectiveness of levocetirizine and desloratadine in up to 4 times conventional doses in difficult-to-treat urticaria, 2010