Sedating antihistamine (phenothiazine) Pregnancy: Avoid in pregnancy unless the physician considers it essential - there is inadequate evidence of safety in human pregnancy. Neuroleptics may occasionally prolong labour and should be withheld until the cervix is dilated 3-4 cm. Possible neonatal effects include lethargy or paradoxical hyperexcitability, tremor and low Apgar score. Phenothiazines may be excreted in milk - breast-feeding should be suspended during treatment (SPC 4.6).

Alimemazine tartrate

Brand names: Vallergan

Alimemazine is a sedating phenothiazine antihistamine, used for short-term symptomatic relief of urticaria and pruritus and as a premedication, and historically for sedation.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 10 mg (approx. 1.7 ml of 30 mg/5 ml syrup)

Route: Oral

Frequency: Two or three times daily

Max: Up to 100 mg per day has been used in intractable cases; do not exceed the recommended dose

Indication in this SPC (Alfresed 30 mg/5 ml Syrup) is urticaria and pruritus. Elderly: dosage should be reduced to 10 mg (approx. 1.7 ml) once or twice daily. Not recommended for infants less than 3 years old and contraindicated in children under 3 years. Children over 3 years should be prescribed the lower-strength 7.5 mg/5 ml syrup for urticaria and pruritus (no per-kg dose stated for that indication). A separate indication, sedation before anaesthesia in children aged 3-7 years, is captured in paedDose. Sections 4.5 and 4.8 were truncated at the source-fetch limit, so the interaction and side-effect lists below may be incomplete.

Paediatric dose

Dose: 2 mg/kg

Route: Oral

Frequency: Single dose 1-2 hours before the operation

Max: 2 mg/kg is stated as the maximum recommended dosage

Applies only to sedation before anaesthesia in children aged 3-7 years. Contraindicated in children under 3 years. For urticaria and pruritus in children over 3 years the SPC directs use of the lower-strength 7.5 mg/5 ml syrup and states no per-kg dose.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Paediatric weight-based calculator

Patient weight (kg)

Verify in a children's formulary

Contraindications

Hypersensitivity to phenothiazines or to any of the excipients
Hepatic or renal dysfunction
Epilepsy
Parkinson's disease
Hypothyroidism
Phaeochromocytoma
Myasthenia gravis
Prostatic hypertrophy
History of narrow angle glaucoma
History of agranulocytosis
Children less than 3 years of age

Side effects

Drowsiness, dizziness, headache
Antimuscarinic effects: dry mouth, constipation, urinary retention, accommodation disorders, nasal congestion
Hypotension (postural hypotension particularly in the elderly or volume-depleted; hypotension or pallor may occur in children)
Extrapyramidal effects: acute dystonias or dyskinesias, akathisia, parkinsonism, tardive dyskinesia; convulsions have been reported
Jaundice, usually transient (withhold treatment if jaundice develops); mild leukopenia on prolonged high dosage, rarely agranulocytosis
Cardiac arrhythmias including atrial arrhythmia, A-V block, ventricular tachycardia and ventricular fibrillation; ECG changes including widened QT interval
Respiratory depression is possible in susceptible patients

Interactions

Alcohol, anxiolytics and hypnotics, opiates, barbiturates and other sedatives - sedative effects of phenothiazines may be intensified (additively)
Tricyclic antidepressants and MAOIs (including moclobemide) - increased antimuscarinic and sedative effects; respiratory depression may occur
Antihypertensive drugs, especially alpha-adrenoceptor blocking agents - hypotensive effect may be exaggerated
Antimuscarinics / other anticholinergic drugs - increased antimuscarinic side effects (constipation, heat stroke); anticholinergic agents may reduce the antipsychotic effect of phenothiazines
Amfetamine, levodopa, clonidine, guanethidine and adrenaline - action may be opposed by phenothiazines

Clinical monograph

How it works

It blocks histamine H1 receptors, with additional antimuscarinic and central depressant actions that account for its sedative effect.

Prescribing in practice

Avoid in young children and use with particular caution given the risk of paradoxical excitation and respiratory depression; it is not recommended for sedation in very young infants.
Sedation may be marked and additive with alcohol and other CNS depressants.
Use cautiously in the elderly and in those with prostatic hypertrophy, glaucoma or hepatic impairment owing to antimuscarinic and sedative effects.

Monitoring

Routine laboratory monitoring is not generally required; review for excessive sedation, antimuscarinic effects and continued need.

Counselling the patient

This medicine can make you drowsy; do not drive or operate machinery if affected.
Avoid alcohol while taking it.
Seek advice before giving to children.

Evidence & guidelines

Use is guided by the SPC and longstanding clinical practice rather than large modern trials.

Reference: SmPC Vallergan; MHRA Drug Safety Update on sedating antihistamines in children; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Urticaria Activity Score (UAS7) · Urticaria

Pathways