Usage
Dimethindene is primarily prescribed for the symptomatic relief of itching associated with various allergic reactions, including:
- Urticaria (hives): A skin rash triggered by allergens, characterized by raised, itchy welts.
- Allergic rhinitis: Inflammation of the nasal passages due to allergens, causing sneezing, runny nose, and itching. This includes hay fever (seasonal allergic rhinitis) and perennial rhinitis (year-round allergic rhinitis).
- Drug and food allergies: Hypersensitivity reactions to specific drugs or foods.
- Atopic dermatitis (eczema): A chronic inflammatory skin condition causing itching, redness, and dryness.
- Pruritus: Itching associated with skin eruptions like chickenpox.
- Insect bites: Localized reactions causing itching and swelling.
Pharmacological Classification: Dimethindene is classified as a first-generation antihistamine, antipruritic, and anticholinergic agent.
Mechanism of Action: Dimethindene acts as a selective histamine H1 receptor antagonist. It competitively binds to H1 receptors, preventing histamine from binding and triggering allergic reactions. This inhibition reduces capillary permeability, thereby decreasing itching, swelling, and redness.
Alternate Names
Dimethindene is also known as dimethindene maleate. A popular brand name is Fenistil.
How It Works
Pharmacodynamics: Dimethindene exerts its antihistaminic effect by blocking the action of histamine at H1 receptors. Its anticholinergic properties contribute to drying effects, such as reduced mucus secretion.
Pharmacokinetics:
- Absorption: Dimethindene is well-absorbed after oral administration.
- Metabolism: It is primarily metabolized in the liver via hydroxylation and methoxylation.
- Elimination: Dimethindene and its metabolites are excreted primarily through the kidneys, with a half-life ranging from 4 to 6 hours. The half-life may be prolonged in patients with hepatic or renal impairment.
Mode of Action: Dimethindene competitively binds to H1 receptors, blocking the binding of histamine. This inhibits histamine-mediated vasodilation, capillary permeability, and smooth muscle contraction, thus relieving allergy symptoms.
Receptor Binding: Selective H1 receptor antagonist.
Elimination Pathways: Primarily renal excretion. Hepatic metabolism by CYP450 enzymes, specifically CYP2D6 and CYP2C19.
Dosage
Standard Dosage
Adults:
Oral: 1-2 mg three times a day. Drops: 20-40 drops three times per day. For patients prone to drowsiness, 40 drops before bedtime and 20 drops in the morning is recommended.
Topical: Apply gel or lotion to the affected area as needed, up to three times a day.
Children:
Oral drops: 0.1 mg/kg body weight per day, divided into three doses (2 drops/kg body weight/day). Adjust dosage based on age and weight:
- 1 month to 1 year: Use with caution and only under medical supervision.
- 1-3 years (15-22.5 kg): 10-15 drops three times a day.
- 3-12 years (22.5-30 kg): 15-20 drops three times a day.
Note: 20 drops = 1 mL = 1 mg dimethindene maleate. 1 drop = 0.05 mg dimethindene maleate.
Topical: Avoid large surface area application in children under 2 years old, especially on broken or inflamed skin. Use with caution in children under 2.
Special Cases:
- Elderly Patients: Initiate with a lower dose and titrate cautiously due to increased sensitivity to anticholinergic effects.
- Patients with Renal Impairment: Dosage adjustment may be necessary.
- Patients with Hepatic Dysfunction: Dosage adjustment may be necessary.
- Patients with Comorbid Conditions: Caution in patients with glaucoma, prostatic hypertrophy, urinary retention, bronchial asthma, chronic obstructive pulmonary disease, or seizures.
Clinical Use Cases
Dimethindene is not typically used for intubation, surgical procedures, mechanical ventilation, or ICU use. Emergency situations are usually treated with other medications.
Dosage Adjustments
Modify dosage based on individual patient factors such as renal/hepatic impairment, age, and other medical conditions. Consult specialized resources for detailed recommendations.
Side Effects
Common Side Effects:
- Drowsiness
- Dry mouth
- Headache
- Nausea
- Rash
Rare but Serious Side Effects:
- Allergic reactions (e.g., skin rash, itching, swelling)
- Paradoxical CNS stimulation (especially in children), including excitability, hallucinations, and seizures
- Difficulty urinating
- Blurred vision
Long-Term Effects:
Tolerance to the sedative effects may develop with prolonged use.
Adverse Drug Reactions (ADR):
Severe allergic reactions (anaphylaxis), significant CNS depression, cardiac arrhythmias.
Contraindications
- Hypersensitivity to dimethindene or any of its components.
- Infants under one month old, particularly premature infants.
- Narrow-angle glaucoma.
- Urinary retention.
- Paralytic ileus.
Drug Interactions
- CNS depressants (e.g., alcohol, sedatives, opioids): Increased sedation and respiratory depression.
- Anticholinergic drugs: Additive anticholinergic effects (dry mouth, constipation, urinary retention).
- MAOIs: Enhanced antihistaminic effects.
- Other antihistamines or decongestants: Additive effects and increased side effects.
- CYP2D6 and CYP2C19 inhibitors or inducers: Altered dimethindene metabolism.
Pregnancy and Breastfeeding
- Pregnancy: Use only if clearly needed. Avoid use on extensive skin surfaces, especially if excoriated or inflamed. Do not apply to nipples during breastfeeding.
- Breastfeeding: Dimethindene is likely excreted in breast milk. Not recommended during breastfeeding.
Drug Profile Summary
- Mechanism of Action: Selective histamine H1 receptor antagonist.
- Side Effects: Drowsiness, dry mouth, headache, nausea, rash. Rarely: allergic reactions, paradoxical CNS stimulation, urinary retention, blurred vision.
- Contraindications: Hypersensitivity, infants under one month, narrow-angle glaucoma, urinary retention, paralytic ileus.
- Drug Interactions: CNS depressants, anticholinergics, MAOIs, other antihistamines, CYP2D6/CYP2C19 inhibitors/inducers.
- Pregnancy & Breastfeeding: Use with caution if clearly needed during pregnancy; not recommended while breastfeeding.
- Dosage: Adults: 1-2 mg or 20-40 drops TID. Children: 0.1mg/kg/day divided TID. Topical: Apply as needed up to TID.
- Monitoring Parameters: Observe for drowsiness, allergic reactions, and other side effects.
Popular Combinations
Not commonly used in fixed combinations. Prescribed as a single agent.
Precautions
- Pre-existing medical conditions (glaucoma, prostatic hypertrophy, urinary retention, asthma, COPD).
- Pregnant and breastfeeding women.
- Infants and young children.
- Elderly patients.
- Avoid alcohol and operating machinery due to potential drowsiness.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for Dimethindene?
A: Adults: 1-2 mg or 20-40 drops TID. Children: 0.1 mg/kg/day divided TID. Topical: Apply as needed up to TID. Elderly and patients with renal/hepatic impairment: Lower doses may be necessary.
Q2: What are the common side effects of Dimethindene?
A: Drowsiness, dry mouth, headache, nausea, rash.
Q3: Can Dimethindene be used in pregnancy?
A: Use only if clearly needed and under medical supervision. Avoid extensive application, especially on damaged skin.
Q4: Is Dimethindene safe for breastfeeding mothers?
A: Not recommended as it is excreted in breast milk.
Q5: How does Dimethindene work?
A: It is a selective H1 receptor antagonist, blocking histamine’s actions and reducing allergic symptoms.
Q6: What are the contraindications for Dimethindene?
A: Hypersensitivity, infants under one month, narrow-angle glaucoma, urinary retention, paralytic ileus.
Q7: Does Dimethindene interact with other medications?
A: Yes, it can interact with CNS depressants, anticholinergics, MAOIs, and other antihistamines, potentially increasing side effects.
Q8: Can I drive while taking Dimethindene?
A: Use caution as it can cause drowsiness. Avoid driving if you experience this side effect.
Q9: What should I do if I miss a dose of Dimethindene?
A: Take the missed dose as soon as you remember. If it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Do not double the dose to catch up.
Q10: Is Dimethindene addictive?
A: Dimethindene is not considered addictive. However, tolerance to the sedative effects can develop with prolonged use.
