1. Clinical Overview
Pheniramine maleate is a first-generation (classical) ethanolamine-derivative antihistamine with pronounced sedative and anticholinergic properties. It is a potent, competitive, and reversible antagonist of the histamine H1 receptor. In the Indian market, it is widely used for symptomatic relief of allergic conditions, primarily as a short-term treatment due to its significant central nervous system (CNS) depressant effects. The 25mg strength is a common adult dose for conditions requiring potent antihistaminic action.
| Onset | Duration | Bioavailability |
|---|---|---|
| 15 to 30 minutes after oral administration. | Approximately 4 to 6 hours. | Not well documented specifically for pheniramine, but for first-gen antihistamines, it is generally high (>50%). Estimated to be 70-80%. |
2. Mechanism of Action
Pheniramine competitively and reversibly blocks the action of histamine at the H1 receptor sites on effector cells (e.g., in the gastrointestinal tract, blood vessels, and respiratory smooth muscle). This blockade inhibits the vasodilatory, bronchoconstrictive, and secretory effects of histamine released during allergic reactions.
3. Indications & Uses
- Symptomatic relief of allergic conditions: Allergic rhinitis (seasonal/perennial)
- Urticaria (hives)
- Allergic conjunctivitis
- Pruritus (itching) of allergic origin
4. Dosage & Administration
Adult Dosage: 25mg to 50mg orally every 4 to 6 hours. Maximum: 150mg per day. The 25mg dose is typical for initiation or in patients sensitive to sedation.
Administration: Administer orally with or without food. Taking with food may reduce GI upset. Tablet should be swallowed whole with a glass of water. Avoid crushing/chewing unless advised. For sleep induction, take 30 minutes before bedtime.
5. Side Effects
Common side effects may include:
- Drowsiness, sedation, somnolence
- Dry mouth, nose, and throat
- Dizziness, disturbed coordination
- Fatigue, lethargy
- Gastrointestinal disturbances (nausea, epigastric distress)
- Thickening of bronchial secretions
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Alcohol, Benzodiazepines, Opioids | Potentiates CNS depression, sedation, and impaired psychomotor performance. | Major |
| Monoamine Oxidase Inhibitors (MAOIs) - e.g., Phenelzine, Tranylcypromine | Increased anticholinergic and CNS depressant effects; risk of hypertensive crisis. | Contraindicated |
| Other Anticholinergics (e.g., Atropine, TCAs, Antipsychotics) | Additive anticholinergic side effects (dry mouth, constipation, urinary retention, confusion). | Major |
| CNS Stimulants (e.g., Amphetamines) | Pheniramine may counteract the stimulant effect. | Moderate |
| Hepatic Enzyme Inhibitors (e.g., Cimetidine, Fluoxetine) | May increase pheniramine plasma levels, increasing toxicity risk. | Moderate |
| Hepatic Enzyme Inducers (e.g., Phenobarbital, Rifampicin) | May decrease pheniramine plasma levels, reducing efficacy. | Moderate |
7. Patient Counselling
- DO take the exact dose prescribed by your doctor.
- DO inform your doctor about all other medicines, including OTC and herbal products.
- DO take with food if stomach upset occurs.
- DONT consume alcohol or sleep-inducing medicines while on this drug.
- DONT drive, operate machinery, or engage in hazardous activities until you know how it affects you.
- DONT take a double dose to make up for a missed one.
- DONT give this medicine to children without pediatrician's advice.
8. Toxicology & Storage
Overdose: Symptoms are primarily an extension of its pharmacological effects: Severe CNS depression (somnolence progressing to coma, respiratory depression) OR paradoxical CNS stimulation (hallucinations, tremors, convulsions). Anticholinergic crisis: fixed dilated pupils, flushing, hyperthermia, dry mucous membranes, tachycardia, hypertension followed by hypotension, urinary retention, ileus. Cardiovascular collapse and death are possible in massive overdose.
Storage: Store below 30°C, in a cool, dry place, protected from light and moisture. Keep the container tightly closed. Keep out of reach and sight of children. Do not use after the expiry date printed on the pack. Do not flush unused medicine; consult pharmacist for safe disposal.