What are the side effects 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.