Upper Respiratory Drugs: First and Second-Generation Antihistamines

Antihistamines block histamine, a chemical released by the immune system during allergic reactions, by competing for H1-receptor binding sites on tissue cells. This blockade reduces inflammation and swelling, alleviating symptoms like sneezing, itching, and nasal congestion. They work by preventing histamine from triggering its inflammatory effects at the cellular level.

First-generation antihistamines cross the blood-brain barrier, causing CNS depression and drowsiness, while second-generation antihistamines have reduced CNS penetration, resulting in less sedation. First-generation drugs also block acetylcholine at muscarinic receptors, producing anticholinergic effects like dry mouth. Second-generation drugs selectively bind to peripheral H1 receptors, making them preferred for daytime use.

First-generation antihistamines can cross the blood-brain barrier into the central nervous system, where they cause CNS depression and induce drowsiness. This occurs because these drugs lack selective peripheral targeting. Their ability to penetrate the CNS makes them less suitable for patients requiring alertness, unlike second-generation alternatives.

First-generation antihistamines block acetylcholine at neuronal and neuromuscular muscarinic receptors, causing anticholinergic side effects including dry mouth, blurred vision, and urinary retention. These effects occur because these drugs interact with cholinergic pathways in addition to blocking histamine. Second-generation antihistamines produce fewer anticholinergic effects due to their selective peripheral binding.

Antihistamines are typically administered orally, making them convenient for self-management of allergies. However, they can also be given via intravenous and intramuscular routes depending on the specific drug and patient condition. Route selection depends on the severity of symptoms and the clinical situation requiring treatment.

First-generation antihistamines include diphenhydramine (Benadryl), chlorpheniramine (Chlor-Trimeton), and promethazine (Phenergan). Second-generation antihistamines include cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra). Second-generation drugs are often preferred due to their reduced sedation and anticholinergic side effects compared to first-generation agents.

Antihistamines, particularly first-generation agents, may cause CNS side effects including fatigue, dizziness, and drowsiness. These effects result from the drugs' ability to cross the blood-brain barrier and depress central nervous system activity. Although antihistamines are generally considered safe, CNS effects should be monitored, and adverse effects should be reported to a healthcare professional.