Antihistamines: What are their side effects?

What are the side effects of antihistamines?

Antihistamines have been used for many years to alleviate allergic reactions. However, there are times when these medications can cause side effects that are more or less bothersome. What are the adverse effects associated with antihistamines? Continue reading to discover them.

Published on November 19, 2024, by Pauline, Chemical Engineer — 5 min of reading

A closer look at the adverse effects associated with the use of antihistamines.

The antihistamines are medications designed to counteract the action of histamine, a chemical mediator produced when the body detects a foreign substance it considers a potentially dangerous intruder. By blocking the histamine receptors, antihistamines prevent this molecule from attaching itself, which reduces the symptoms of allergic reactions. However, it is important to know that these medications only have a short-term preventive effect : they do not treat the allergy but alleviate its effects. Antihistamines are generally taken once a day, orally or nasally. When the symptoms disappear, it is possible to stop the treatment directly.

Antihistamines are generally well tolerated by the body. However, depending on their type, it is possible to experience certain adverse effects.

The presence or absence of side effects can vary from one medication to another, depending on whether it is an antihistamine H1 or H2. The former are generally associated with risks of drowsiness and a decrease in alertness. These effects can hinder productivity at school as well as at work. Driving after taking an antihistamine is indeed discouraged, as are other high-risk activities, such as operating a dangerous machine. However, it is not uncommon for this initial slight sedation to gradually fade as one becomes accustomed to the antihistamines. This is not automatic, however, and it is still recommended to seek medical advice in case of drowsiness following the intake of these medications.

Besides this, in rare instances, H1 antihistamines can stimulate appetite or decrease psychomotor performance. H1 antihistamines are also likely to have an anticholinergic effect, meaning they can block peripheral cholinergic receptors. These are transmembrane proteins capable of binding acetylcholine released into the extracellular environment and subsequently inducing a signal within the cytoplasm. This potential anticholinergic effect of antihistamines can result in dry mouth, urinary retention, accommodation disorders, mental confusion, or even tachycardia. However, these discomforts are rare and are often caused by an overdose, hence the importance of adhering to the doses prescribed by healthcare professionals.

Second-generation H1 antihistamines, such as levocetirizine, desloratadine, fexofenadine, bilastine, or rupatadine, have an almost negligible risk of side effects, unlike those of the first generation, such as diphenhydramine, clemastine, or levomepromazine.

As for H2 antihistamines, they are typically used in cases of stomach or duodenal ulcers, or gastroesophageal reflux issues. They too can cause unpleasant side effects, such as diarrhoea, a skin rash, fever, muscle pain or confusion. The H2 receptor antagonist cimetidine can also cause an increase in breast size and erectile dysfunction in men. However, these side effects are relatively rare.

Antihistamines: what precautions should be taken?

In addition to strictly adhering to the dosage prescribed by the doctor, antihistamines are subject to various contraindications and usage precautions. Indeed, first-generation H1 antihistamines are contraindicated for people engaged in activities requiring continuous vigilance, such as driving, due to their sedative effect. These medications should also not be used by patients suffering from closed-angle glaucoma, benign prostatic hypertrophy, or urinary retention as they can exacerbate these conditions due to their anticholinergic action. Elderly people or those with cardiovascular diseases are also advised to exercise caution, as first-generation H1 antihistamines can interfere with cardiac conduction. Although second-generation ones are safer, it is recommended to remain vigilant.

As for H2 antihistamines, they are well tolerated, whether by children, the elderly, or pregnant women. However, it is advised not to combine them with posaconazole, an antifungal, as this could reduce its absorption. On the other hand, it is preferable to take antihistamines away from antacids, medications often prescribed in cases of acid reflux, so as not to impair their effectiveness.