Usage
Clemastine is an antihistamine, specifically an H1-receptor antagonist, primarily prescribed for the symptomatic relief of allergic rhinitis (hay fever), urticaria (hives), and other allergic reactions. It is effective in reducing sneezing, runny nose, itching, watery eyes, and skin rash associated with these conditions. It can also provide temporary relief from rhinorrhea and sneezing related to the common cold.
Alternate Names
While “clemastine” or “clemastine fumarate” is the generic name, it’s marketed under various brand names such as Tavist and Allerhist-1.
How It Works
Pharmacodynamics: Clemastine competitively inhibits the binding of histamine to H1 receptors. This action prevents histamine-mediated vasodilation, capillary permeability increase, and smooth muscle contraction, thus alleviating allergy symptoms.
Pharmacokinetics:
- Absorption: Clemastine is well-absorbed orally.
- Metabolism: Primarily metabolized in the liver.
- Elimination: Mainly excreted through urine.
Mode of Action: Clemastine acts as an inverse agonist at the H1 receptor. It binds to the receptor and stabilizes it in an inactive conformation, thereby reducing the basal activity of the receptor and preventing histamine from exerting its effects.
Dosage
Standard Dosage
Adults:
- Allergic Rhinitis and Urticaria: 1.34 mg orally twice daily. The dosage may be increased to a maximum of 2.68 mg three times a day, but not to exceed 8.04 mg/day. Some patients respond well to a single dose of 2.68 mg, repeated as needed, but not exceeding three tablets daily.
- Common Cold: 1.34 mg orally twice daily, not exceeding 2.68 mg/day for self-medication. Higher doses may be directed by a clinician.
Children:
- <6 years: 0.335-0.67 mg/day divided into two or three doses, with a maximum daily dose of 1.34 mg.
- 6-12 years: 0.67-1.34 mg twice daily, maximum daily dose of 4.02 mg.
- >12 years: As in adults.
Special Cases:
- Elderly Patients: Lower doses are recommended, starting with 1.34 mg twice daily, due to increased sensitivity to anticholinergic effects.
- Patients with Renal Impairment: Dose adjustment may be necessary.
- Patients with Hepatic Dysfunction: Dose adjustment may be necessary.
- Patients with Comorbid Conditions: Caution is advised in patients with asthma, glaucoma, hyperthyroidism, cardiovascular disease, or hypertension.
Clinical Use Cases
Clemastine is not typically used in clinical settings like intubation, surgical procedures, mechanical ventilation, or the ICU. It is primarily indicated for the outpatient management of allergic conditions and the common cold. For anaphylaxis, intravenous formulations of H1 antihistamines like dimetindene or clemastine are preferred.
Side Effects
Common Side Effects
Drowsiness, dizziness, dry mouth, blurred vision, constipation, and gastrointestinal upset.
Rare but Serious Side Effects
Allergic reactions (hives, difficulty breathing, swelling), seizures, hallucinations, difficulty urinating, easy bruising or unusual bleeding, and irregular heartbeat.
Contraindications
Hypersensitivity to clemastine. Use with caution in patients with narrow-angle glaucoma, urinary retention, prostatic hypertrophy, stenosing peptic ulcer, pyloroduodenal obstruction, or bladder neck obstruction.
Drug Interactions
Alcohol, other CNS depressants (sedatives, hypnotics), and monoamine oxidase inhibitors (MAOIs) can potentiate the sedative effects of clemastine.
Pregnancy and Breastfeeding
Clemastine is generally considered safe during pregnancy, but it’s advisable to consult a physician. It may pass into breast milk and could affect the nursing infant.
Drug Profile Summary
- Mechanism of Action: H1 receptor antagonist (inverse agonist).
- Side Effects: Drowsiness, dizziness, dry mouth, blurred vision, constipation. Serious side effects include allergic reactions, seizures, hallucinations.
- Contraindications: Hypersensitivity, narrow-angle glaucoma, urinary retention, prostatic hypertrophy.
- Drug Interactions: Alcohol, CNS depressants, MAOIs.
- Pregnancy & Breastfeeding: Generally safe during pregnancy; consult a physician. May pass into breast milk.
- Dosage: Adults: 1.34 mg twice daily, up to 2.68 mg three times daily. Children: Dose varies by age.
- Monitoring Parameters: Observe for adverse reactions, especially in elderly patients.
Popular Combinations
Clemastine is not typically used in combinations with other drugs for its primary indications.
Precautions
Use cautiously in patients with pre-existing respiratory or cardiovascular conditions. Avoid alcohol and other CNS depressants. Caution is needed when operating machinery or driving due to potential drowsiness.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for Clemastine?
A: Adults: 1.34 mg twice daily, adjustable up to 2.68 mg three times daily. Children: Dosage varies depending on age, weight, and condition.
Q2: What are the common side effects of Clemastine?
A: Drowsiness, dry mouth, dizziness, blurred vision, and constipation.
Q3: Can Clemastine be used during pregnancy?
A: Generally considered safe, but consult a physician before use.
Q4: Does Clemastine interact with other medications?
A: Yes, it can interact with alcohol, CNS depressants (like sedatives or sleeping pills), and MAOIs.
Q5: How does Clemastine work?
A: It blocks the action of histamine at H1 receptors, reducing allergy symptoms.
Q6: What conditions is Clemastine used to treat?
A: Primarily allergic rhinitis, urticaria, and symptoms of the common cold.
Q7: Are there any contraindications for Clemastine use?
A: Yes, contraindications include hypersensitivity to clemastine, certain medical conditions like narrow-angle glaucoma, and concomitant use of MAOIs.
Q8: Can Clemastine be used in elderly patients?
A: Yes, but with caution and at a lower starting dose due to increased risk of side effects.
Q9: What should patients be advised about taking Clemastine?
A: Patients should be advised about potential side effects like drowsiness, avoid alcohol and other CNS depressants, and use caution when operating machinery. They should also inform their physician about any other medications they are taking.
