Terfenadin

Uses

Terfenadin is used for the treatment of allergic rhinitis, hay fever, and allergic skin disorders

Terfenadin is an antihistamine. Antihistamines prevent sneezing, runny nose, itching and watering of the eyes, and other allergic symptoms. This is used to treat allergies, hives (urticaria), and other allergic inflammatory conditions. Terfenadin may also be used for purposes other than those listed in this medication guide.

Terfenadin is also used to associated treatment for these conditions: Allergic Rhinitis (AR), Allergies, Allergy Skin, Pollen Allergy

How Terfenadin works

Terfenadin competes with histamine for binding at H1-receptor sites in the GI tract, uterus, large blood vessels, and bronchial muscle. This reversible binding of terfenadine to H1-receptors suppresses the formation of edema, flare, and pruritus resulting from histaminic activity. As the drug does not readily cross the blood-brain barrier, CNS depression is minimal.

Dosage

Terfenadin dosage

Adult (>12 year and >50 kg): 60-120 mg in the morning or 60 mg bid. Max: 120 mg daily.

Renal Impairment: CrCl <40: Half the usual daily dose

Side Effects

Anxiety, palpitations, insomnia, mild GI distubances, erythema multiforme and galactorrhoea.

Toxicity

Mild (e.g., headache, nausea, confusion), but adverse cardiac events including cardiac arrest, ventricular arrhythmias including torsades de pointes and QT prolongation have been reported. LD₅₀=mg/kg (orally in mice)

Precaution

Child and elderly. Avoid in patients with cardiac or significant hepatic disease, electrolyte imbalance, or known or suspected prolongation of the QT interval. Lactation, pregnancy.

Interaction

Increased risk of ventricular arrhythmia with triazole and imidazole antifungals, macrolide antibacterials, streptogramin antibacterials, SSRI, HIV-protease inhibitors and non-nucleoside reverse transcriptase inhibitors, arrhythmogenic drugs, diuretics, astemizole and zileutron.

Food Interaction

• Exercise caution with grapefruit products. Grapefruit is a CYP3A4 inhibitor, and terfenadine is metabolized by CYP3A4. Co-administration may increase terfenadine concentrations and the risk of QT prolongation.
• Exercise caution with St. John's Wort. This herb induces CYP3A4 and terfenadine is a substrate of CYP3A4.

[Major] CONTRAINDICATED: The consumption of grapefruit juice has been associated with significantly increased plasma concentrations of terfenadine.

The mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall induced by certain compounds present in grapefruits.

Terfenadin in high serum levels has been associated with prolongation of the QT interval and development of torsade de pointes, a potentially fatal ventricular arrhythmia.

MANAGEMENT: Due to the risk of cardiotoxicity, patients receiving the drug should be advised to avoid consumption of grapefruit products.

Loratadine, cetirizine, and fexofenadine may be safer alternatives in patients who may have trouble adhering to the dietary restriction.

Terfenadin Drug Interaction

Moderate: halothaneUnknown: acetaminophen / diphenhydramine, aspirin, charcoal, amphetamine / dextroamphetamine, epinephrine, glimepiride, amobarbital, aspirin, amoxicillin / clavulanate, penicillin g benzathine, methohexital, remimazolam, dexamethasone, dextromethorphan, glucose, alvimopan, isoflurane, acetaminophen, phytonadione

Elimination Route

On the basis of a mass balance study using 14C labeled terfenadine the oral absorption of terfenadine was estimated to be at least 70%

Half Life

3.5 hours

Pregnancy & Breastfeeding use

Pregnancy Category C. Either studies in animals have revealed adverse effects on the foetus (teratogenic or embryocidal or other) and there are no controlled studies in women or studies in women and animals are not available. Drugs should be given only if the potential benefit justifies the potential risk to the foetus.

Contraindication

Porphyria.

Innovators Monograph

You find simplified version here Terfenadin