How to use antihistamines

This article is welcome, but the authors miss the point on several issues.

First: Antihistamines are commonly and incorrectly used, largely because they are bought off the shelf in pharmacies, and professionals – whether physicians or pharmacists – have little influence on how they are used. Most products on the shelves contain diphenhydramine: very sedating. They are marketed for the purposes that this article questions: insomnia, cough, and eczema. People keep buying them because they perceive them as effective, without good alternatives. We need to listen to the customers more. In particular, while antihistamines are ineffective against many infectious coughs, they treat the “clearing the throat” cough that often accompanies allergic rhinitis, and may be a pathognomonic sign.

Second: First generation antihistamines are associated with serious side effects, but there is no clear distinction between the first and later generation drugs, but a gradation. Among the three available in Canada, diphenhydramine and hydroxyzine are most sedating, while chlorpheniramine is less so. The quoted half-lives are based on cutaneous wheal and flare reactions, but many patients find the sedating effect of diphenhydramine 25mg lasts up to 12 hours, while sedative effects of chlorpheniramine 4mg are not perceptible on wakening in the morning. Among the second-generation antihistamines the WHO review1 shows that cetirizine is more sedating, but it is unclear whether there is a difference among the others2.

The anticholinergic and adrenergic effects of first generation drugs are useful for many patients with allergic rhinitis, since they reduce nasal congestion. Obstructed breathing, with concomitant snoring and poor sleep, diminishes; so during allergy season many of my patients find this improves their sleep and hence alertness next day. While sedation is a problem during the day, most people find it has worn off by the morning after low dose chlorpheniramine is taken at night.

Third: Newer antihistamines are safer and efficacious. However, in most Canadian pharmacies, they are not as affordable as the older ones. The authors’ price comparisons are incomplete, ignoring chlorpheniramine. This less sedating and useful drug sells in bottles of 100 for $10 to 15 – substantially less than others. This matters for many patients who use them regularly.

Evidence on older generation antihistamines is limited, as they were marketed before modern expectations on evidence were developed. Nonetheless, absence of evidence does not mean lack of effectiveness. The authors are right to condemn the widespread sale of diphenhydramine for different indications. It should indeed be limited in availability3, or sold in lower dose versions that would be less dangerous.

Physicians should carefully listen to their patients, to help them select and adjust their medications to suppress allergic rhinitis. Both first and second-generation antihistamines are useful.