FENISTIL DROPS

Indications

• Allergic rhinitis
• Urticaria
• Allergic conjunctivitis
• Pruritus

Dosage

Children: For children aged 1 to 3 years, the recommended dose is 0.25 mg twice daily. For children aged 3 to 12 years, the recommended dose is 0.5 mg to 1 mg twice daily, depending on the severity of symptoms. Refer to the BNF for Children for specific recommendations.

Adults: The usual adult dose is 1-2 mg taken orally before bedtime, which may be adjusted based on clinical response.

Mechanism of action

Dimethindene is a selective histamine H1 antagonist that binds to the histamine H1 receptor. This binding inhibits the action of endogenous histamine, leading to temporary relief from the symptoms associated with its release.

Pharmacodynamics

Dimethindene exists as a racemic mixture. The (S)-(+)-dimethindene acts as a potent M2-selective muscarinic receptor antagonist, while the (R)-(-)-enantiomer is the eutomer that is responsible for the drug's bioactivity at the histamine H1 receptor. This unique property allows it to provide effective relief from allergic reactions.

Pharmacokinetics

Dimetindenum is well absorbed following oral administration, and its effects can be noted within one hour. It undergoes hepatic metabolism and is primarily excreted via urine. The drug has a relatively long half-life, which supports its dosing schedule.

Contra-indications

• Hypersensitivity to dimethindene or any of its components
• Severe asthma
• Newborns and premature infants

Adverse effects

• Drowsiness
• Dry mouth
• Dizziness
• Gastrointestinal disturbances
• Blurred vision
• Headache

Interactions

• Alcohol - may enhance sedative effects
• CNS depressants - may potentiate drowsiness and sedation
• Anticholinergic drugs - may increase anticholinergic effects

Precautions

• Use with caution in patients with glaucoma
• Use with caution in patients with prostatic hypertrophy
• Caution in patients with epilepsy or a history of seizures
• Should be used with caution in the elderly due to increased sensitivity

Pregnancy

Safety in pregnancy has not been established. Use only if the potential benefit justifies the potential risk to the fetus.

Breast-feeding

Dimethindene is excreted in breast milk. Caution is advised when administered to nursing mothers.

Storage

Store in a cool, dry place away from light. Keep out of reach of children.

Formulations

• Oral solution
• Tablets

Mechanism of action

Dimethindene is a selective histamine H1 antagonist and binds to the histamine H1 receptor. This blocks the action of endogenous histamine, which subsequently leads to temporary relief of the negative symptoms brought on by histamine.

Pharmacodynamics

Dimethindene occurs as a racemic mixture. The (S)-(+)-dimethindene is a potent M2-selective muscarinic receptor antagonist (with lower affinity for M1, M3, and M4 muscarinic receptors). The (R)-(-)-enantiomer is the eutomer (responsible for bioactivity) for histamine H1 receptor binding.