Loratadine and Second-Generation Antihistamines Explained

Antihistamines are among the most widely used medicines in the world, found in nearly every home medicine cabinet and pharmacy. They are used to relieve symptoms of allergies — hay fever (allergic rhinitis), hives (urticaria), insect bites, and other conditions in which histamine plays a role. The class divides neatly into two generations, and the distinction matters: it explains why some antihistamines make you drowsy while others are marketed as “non-drowsy.” Loratadine, one of the best-known second-generation antihistamines, sits at the centre of this story and was one of the first to move from prescription-only to widespread over-the-counter (OTC) availability.

The story is partly pharmacological and partly regulatory. Understanding how these medicines differ — and why their names and brands look the way they do — helps make sense of a shelf that can otherwise seem bewildering.

First-generation antihistamines: the original H1 blockers

Histamine is a chemical released by certain cells in the body during an allergic reaction. It binds to histamine H1 receptors, producing the familiar symptoms of allergy: itching, sneezing, a runny nose, and watery eyes. The first antihistamines to reach the market, from the 1940s onward, block these H1 receptors but are also small and lipophilic (fat-soluble) enough to cross the blood-brain barrier readily. In the central nervous system they produce sedation and drowsiness.

These older medicines — which include chlorphenamine (also written chlorpheniramine), diphenhydramine, promethazine, and hydroxyzine — also bind to other receptor types, notably muscarinic acetylcholine receptors. This “anticholinergic” effect accounts for side effects such as dry mouth, blurred vision, constipation, and urinary retention. Their duration of action is generally short, often requiring several doses a day. Despite these drawbacks, first-generation antihistamines remain useful in certain settings and appear on the World Health Organization’s Essential Medicines List (EML), where chlorphenamine is listed as an example of the class.

The second-generation shift

Second-generation antihistamines were developed from the 1980s onward to reduce the central nervous system effects that limited the older drugs. They are more selective for H1 receptors and are designed to be less lipophilic, so they cross the blood-brain barrier far less readily. The practical result is significantly less sedation at standard doses — which is why many are marketed as “non-drowsy” — and far fewer anticholinergic effects. Members of this group include loratadine, cetirizine, fexofenadine, desloratadine, levocetirizine, and bilastine. They also tend to have longer half-lives than the first-generation drugs, meaning a single dose is active for longer. The WHO EML includes cetirizine and loratadine as representative second-generation antihistamines.

It is worth noting that “non-drowsy” is a relative term. Some individuals, particularly at higher doses, may still experience drowsiness with second-generation agents, and cetirizine is sometimes described as mildly more sedating than loratadine or fexofenadine. The improvement over the first generation is clear, but the drugs are not stimulants; they simply cause less central nervous system depression in most people.

Names, brands, and metabolites

The international nonproprietary names (INNs) of antihistamines follow recognisable patterns. Loratadine shares its “-tadine” ending with desloratadine, which is its pharmacologically active metabolite — the compound the body converts loratadine into. Cetirizine and levocetirizine are similarly related: levocetirizine is the active enantiomer (one of the two mirror-image forms) of cetirizine. Fexofenadine, in turn, is the active metabolite of terfenadine, an earlier second-generation drug that was withdrawn from the market because of cardiac safety concerns involving irregular heart rhythms.

Brand names are far more familiar to the public than INNs. Loratadine was originally launched by Schering-Plough as Claritin. Cetirizine was marketed as Zyrtec. Fexofenadine is sold as Allegra in the United States and Telfast in several other markets. As patents expired, generic versions flooded the market, and today loratadine and cetirizine are sold under dozens of store and house brands alongside the original trade names. The active ingredient is identical; the brand is a commercial distinction.

OTC status around the world

One of the defining features of the second-generation antihistamines is their widespread availability without a prescription. Loratadine was a prescription-only medicine when first approved by the US Food and Drug Administration (FDA) in 1993, then switched to OTC status in 2002. Cetirizine followed a similar path, going OTC in the United States in 2007. In the European Union, many second-generation antihistamines have been available in pharmacies without a prescription for years, often in pack sizes suitable for short-term use.

Regulatory scheduling varies by country. In South Africa, the scheduling of antihistamines under the Medicines and Related Substances Act determines whether a product can be sold on a general shop shelf, in a pharmacy with pharmacist advice, or only on prescription. Loratadine and several other second-generation antihistamines are typically available at lower schedules, reflecting their wider safety margin compared with first-generation agents. SAHPRA (the South African Health Products Regulatory Authority) maintains a publicly searchable register of registered medicines, and tools such as HealthSA can be used to look up registration status and the Single Exit Price for specific products. In many low- and middle-income countries, access still depends on local registration and supply; the inclusion of antihistamines on the WHO EML signals that health authorities consider them essential for basic healthcare systems, but actual availability on pharmacy shelves is shaped by national regulation, importation rules, and market interest.