Mechanism of action
Loratadine functions as a selective inverse agonist at peripheral H1 histamine receptors. By binding to H1 receptors, it stabilizes them in an inactive conformation and prevents histamine from triggering the allergic inflammatory cascade. This blockade reduces vasodilation, capillary permeability, and pruritus mediated by histamine release from mast cells and basophils.
The key distinguishing feature of loratadine and other second-generation antihistamines is their peripheral selectivity. Loratadine is a lipophilic molecule, but it does not readily cross the blood-brain barrier due to strong P-glycoprotein efflux at the blood-brain barrier, minimizing central H1 receptor antagonism. This accounts for the dramatically reduced sedation compared to first-generation agents like diphenhydramine. Additionally, loratadine exhibits negligible affinity for muscarinic cholinergic receptors, alpha-adrenergic receptors, and serotonin receptors, avoiding the anticholinergic and cardiovascular side effects seen with older antihistamines.
Loratadine is a prodrug that undergoes extensive first-pass metabolism to desloratadine (descarboethoxyloratadine), an active metabolite with greater H1 receptor affinity and a longer half-life. Both the parent compound and desloratadine contribute to the clinical antihistamine effect.
- Loratadine is a second-generation H1 antihistamine with minimal sedation due to poor CNS penetration from P-glycoprotein efflux.
- It is a prodrug metabolized by CYP3A4 and CYP2D6 to the active metabolite desloratadine.
- Unlike terfenadine and astemizole, loratadine does not prolong the QT interval.
- Loratadine lacks anticholinergic effects, avoiding dry mouth, urinary retention, and constipation seen with first-generation agents.
- Most common adverse effect is headache; sedation occurs in fewer than 8% of patients.
- Approved for allergic rhinitis (seasonal and perennial) and chronic idiopathic urticaria.
- Once-daily dosing with onset of action within 1–3 hours and duration of 24 hours.
- Safe in pregnancy (former category B) and generally compatible with breastfeeding.
Indications
Loratadine is FDA-approved for the relief of symptoms associated with:
- Allergic rhinitis (seasonal and perennial): sneezing, rhinorrhea, nasal pruritus, and itching of the eyes, palate, and throat
- Chronic idiopathic urticaria: reduction of pruritus and number and size of hives
Off-label and clinical uses include symptomatic relief in mild allergic conjunctivitis and adjunctive treatment in atopic dermatitis to control pruritus. Loratadine is often preferred in patients who require daytime alertness, such as students and those operating machinery, due to its non-sedating profile.
Pharmacokinetics
Absorption: Loratadine is rapidly absorbed from the gastrointestinal tract, with peak plasma concentrations reached within 1 to 2 hours. Food increases the bioavailability slightly but does not significantly affect clinical efficacy.
Distribution: The volume of distribution is large, and loratadine is highly protein-bound (97–99%). Its lipophilic nature combined with substrate affinity for P-glycoprotein efflux pumps limits CNS penetration, maintaining its peripheral selectivity.
Metabolism: Loratadine undergoes extensive first-pass hepatic metabolism primarily via CYP3A4 and CYP2D6 to form the active metabolite desloratadine. Desloratadine has equal or greater antihistaminic activity and a half-life of approximately 27 hours, compared to loratadine's half-life of about 8 hours.
Excretion: Elimination occurs through both renal and biliary routes, with approximately 40% excreted in urine and 40% in feces over 10 days. The effective duration of action is 24 hours, permitting once-daily dosing.
Special populations: In patients with hepatic impairment or renal insufficiency (creatinine clearance <30 mL/min), the elimination half-life is prolonged, and dose adjustment may be warranted. The drug is not effectively removed by hemodialysis.
Adverse effects
Loratadine is generally well tolerated, and the incidence of adverse effects is comparable to placebo in many clinical trials. The most commonly reported adverse effects include:
- Headache (most frequent, ~12% incidence)
- Fatigue or somnolence (less common than first-generation agents, ~4–8%)
- Dry mouth (xerostomia, mild, ~3%)
- Gastrointestinal symptoms: nausea, dyspepsia
- Nervousness or dizziness (rare)
Sedation: Although classified as non-sedating, a small subset of patients (~5–8%) may experience mild somnolence, particularly at higher-than-recommended doses or when combined with CNS depressants.
Cardiovascular: Unlike terfenadine and astemizole (withdrawn antihistamines), loratadine does not prolong the QT interval at therapeutic doses and is not associated with torsades de pointes.
Hypersensitivity: Rare reports of anaphylaxis, angioedema, and rash exist.
Hepatotoxicity: Isolated case reports of transient liver enzyme elevations have been documented but are exceedingly rare.
Contraindications and warnings
Contraindications:
- Known hypersensitivity to loratadine or desloratadine
Warnings and precautions:
- Hepatic impairment: Patients with significant liver disease may have reduced metabolism and should start with a lower dose or extended dosing interval.
- Renal impairment: Dose adjustment is recommended in patients with creatinine clearance below 30 mL/min.
- Pregnancy: Loratadine is FDA pregnancy category B (now replaced by the Pregnancy and Lactation Labeling Rule). Animal studies have not demonstrated fetal harm, but human data are limited. It is generally considered acceptable during pregnancy when benefits outweigh risks, particularly in the second and third trimesters.
- Lactation: Loratadine and its metabolite are excreted in breast milk in small amounts. While generally considered compatible with breastfeeding, monitoring the infant for irritability or drowsiness is advised.
- Phenylketonuria: Some orally disintegrating tablet formulations contain phenylalanine and should be avoided in patients with PKU.
Pediatric use: Loratadine is approved for use in children aged 2 years and older. Safety and efficacy in children under 2 years have not been established.
Drug interactions
Loratadine has a relatively low potential for clinically significant drug interactions compared to first-generation antihistamines.
CYP3A4 and CYP2D6 inhibitors: Drugs that inhibit these enzymes can increase plasma concentrations of loratadine and desloratadine. Clinically significant interactions have been observed with:
- Ketoconazole: increases loratadine AUC but does not increase QT prolongation or adverse CNS effects
- Erythromycin: similar interaction, generally well tolerated
- Cimetidine: increases loratadine levels modestly
These interactions rarely necessitate dose adjustment due to loratadine's wide therapeutic index.
CNS depressants: Although loratadine is minimally sedating, concurrent use with alcohol, benzodiazepines, opioids, or other sedatives may have additive CNS depressant effects in sensitive individuals.
Anticholinergic drugs: Loratadine has minimal anticholinergic activity, but additive effects are theoretically possible with drugs like tricyclic antidepressants or antimuscarinics.
P-glycoprotein substrates: Loratadine is a substrate of P-glycoprotein efflux pumps; inhibitors (e.g., cyclosporine, verapamil) may increase CNS penetration, though this is not commonly clinically significant.
Clinical pearls
- Non-sedating profile: Loratadine is preferred over first-generation antihistamines (diphenhydramine, chlorpheniramine) in patients needing to maintain alertness. However, second-generation does not mean zero sedation—counsel patients to assess individual response.
- Prodrug metabolism: The active metabolite desloratadine is available as a separate drug and has a longer half-life, but therapeutic differences are minimal in most patients.
- Once-daily dosing: Excellent adherence due to convenient dosing and OTC availability in many countries.
- Does not prolong QT: Unlike older second-generation agents (terfenadine, astemizole), loratadine is safe in patients at risk for arrhythmias.
- Rapid onset: Symptom relief typically begins within 1–3 hours, with maximum effect at 8–12 hours.
- Exam tip: Loratadine is the classic example of a second-generation antihistamine used to contrast with first-generation agents. Remember: second-generation = lipophilic but P-glycoprotein efflux limits CNS penetration, resulting in less sedation and no anticholinergic effects.
- Alternative in sedation-sensitive patients: Preferred in elderly patients, pilots, drivers, and those on other CNS-active medications.
- Seasonal use: Can be started before allergen exposure (e.g., pollen season) for prophylactic benefit.
Second-generation antihistamines mnemonic: CALM — Cetirizine, Acrivastine, Loratadine, (Fexo)fenadine = non-sedating, peripherally selective H1 blockers.
Practice questions
1. A 28-year-old woman presents to the clinic with sneezing, nasal congestion, and itchy, watery eyes that began 3 days ago during a camping trip. She has a history of seasonal allergies but has not taken medication in the past year. She works as a commercial airline pilot and is concerned about sedation. Physical examination reveals clear nasal discharge and conjunctival injection. Which of the following is the most appropriate pharmacotherapy?
2. A 45-year-old man with chronic urticaria is started on loratadine. He is also taking ketoconazole for a fungal infection and is concerned about drug interactions after reading about antihistamines online. Which of the following statements is most accurate regarding this drug combination?
3. A 5-year-old boy is brought to the pediatrician for evaluation of perennial allergic rhinitis. His mother reports that he has daily nasal congestion, sneezing, and eye itching that interfere with sleep and school performance. She requests a non-drowsy medication. The physician prescribes loratadine. What is the primary reason this drug causes less sedation than first-generation antihistamines?
Frequently asked questions
How does loratadine work?
Loratadine works by selectively blocking peripheral H1 histamine receptors, preventing histamine from causing allergic symptoms like sneezing, itching, and hives. It is a second-generation antihistamine that does not readily cross the blood-brain barrier, so it causes minimal sedation. Loratadine is metabolized to desloratadine, an active metabolite that contributes to its 24-hour duration of action.
What are the side effects of loratadine?
Loratadine is well tolerated, with the most common side effect being headache, occurring in about 12% of patients. Unlike first-generation antihistamines, it causes minimal sedation (fewer than 8% of users) and has negligible anticholinergic effects. Other mild side effects include dry mouth, fatigue, nausea, and dizziness. Serious adverse effects such as allergic reactions or liver toxicity are rare.
Is loratadine sedating or non-sedating?
Loratadine is classified as a non-sedating antihistamine because it does not readily cross the blood-brain barrier due to P-glycoprotein efflux pumps that actively transport it out of the CNS. However, a small percentage of patients (5–8%) may experience mild drowsiness, especially at higher doses or when combined with alcohol or other CNS depressants. It is significantly less sedating than first-generation antihistamines like diphenhydramine.
Can you take loratadine during pregnancy?
Loratadine is considered relatively safe during pregnancy and was classified as FDA pregnancy category B before the system was updated. Animal studies have not shown fetal harm, and it is often preferred over first-generation antihistamines due to its better safety profile and lack of sedation. However, it should be used only when benefits outweigh potential risks, and consultation with a healthcare provider is recommended, especially in the first trimester.
Loratadine vs cetirizine — what is the difference?
Both loratadine and cetirizine are second-generation antihistamines used for allergic rhinitis and urticaria, but cetirizine is slightly more sedating due to modest CNS penetration in some patients. Cetirizine has a faster onset of action (about 20–60 minutes) compared to loratadine (1–3 hours). Both are dosed once daily, but cetirizine may be more effective for severe itching. Loratadine is metabolized by CYP3A4 and CYP2D6, while cetirizine is minimally metabolized, making drug interactions less of a concern with cetirizine.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: R06AX13
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.