Dexamfetamine

Dexamfetamine is the dextro isomer of amfetamine, a central nervous system stimulant used for attention deficit hyperactivity disorder and narcolepsy. It is a controlled drug with a high potential for misuse and dependence. It is used in specialist care, particularly when methylphenidate has failed or is not tolerated.

Classification. Central nervous system stimulant, amfetamine.

Mechanism. Releases dopamine and noradrenaline from nerve terminals and blocks their reuptake, increasing synaptic levels.

Pharmacokinetics. Well absorbed orally, peak in about 2 to 3 hours. Partly metabolised by CYP2D6 and excreted in urine, with excretion higher in acid urine. Half-life about 10 to 12 hours, with an effect lasting 4 to 6 hours for immediate release.

All regimens are oral with no loading dose.

ADHD, children aged 6 years and over.

• Start 2.5 mg once or twice daily (5 mg once or twice daily in some labels).
• Titration: increase by 2.5 to 5 mg at weekly intervals.
• Usual 10 to 20 mg daily in two or three divided doses. Maximum 20 mg daily (up to 40 mg in some labels).

ADHD, adults (specialist use).

• Start 5 mg once or twice daily.
• Titration: increase by 5 mg weekly.
• Usual 20 to 40 mg daily in divided doses. Maximum 60 mg daily.

Narcolepsy. Children 6 to 12 years: start 5 mg daily and increase by 5 mg weekly. Children over 12 years and adults: start 10 mg daily and increase by 10 mg weekly. Usual 5 to 60 mg daily in divided doses.

Dose limits. Review treatment at least yearly and consider a break. Kidney and liver impairment: use with caution and lower doses.

Contraindications. Concurrent monoamine oxidase inhibitor or within 14 days of stopping one, severe hypertension, symptomatic cardiovascular disease, arrhythmia, hyperthyroidism, glaucoma, agitated states, history of drug misuse, hypersensitivity. Major warning of abuse, misuse and addiction.

Serious adverse effects. Hypertension, tachycardia and arrhythmias, cardiomyopathy with prolonged use, sudden death with structural heart disease, psychosis or mania, seizures, peripheral vasculopathy, growth slowing in children, dependence and misuse, serotonin syndrome.

Common adverse effects. Reduced appetite, weight loss, insomnia, headache, dry mouth, irritability, tachycardia.

Interactions. Monoamine oxidase inhibitors are contraindicated. Serotonergic medicines add serotonin syndrome risk. Medicines that acidify urine lower levels, and alkalinising agents raise levels. Khat (cathinone) and other sympathomimetics add cardiovascular and psychiatric effects. Antihypertensive effect may be reduced.

Common prescribing errors. Starting at too high a dose; late-day dosing; prescribing without cardiac assessment; large supplies in patients at risk of diversion; omitting height and weight records.

Special groups.

• Pregnancy: avoid unless the benefit clearly outweighs the risk; fetal growth effects reported.
• Breastfeeding: passes into milk; avoid.
• Children: not for children under 6 years for ADHD; monitor growth.
• Older adults: limited data.
• Kidney impairment: use lower doses.
• Liver impairment: use lower doses.

Formulations. Tablets 5 mg; oral solution 5 mg/5 mL in some regions; modified-release capsules in some regions. Give the first dose on waking and later doses at 4 to 6 hour intervals, with the last dose by early afternoon.

Baseline. Blood pressure, pulse, height and weight, cardiovascular history and family history of sudden cardiac death, psychiatric history, substance use including khat and alcohol.

Monitoring. Check blood pressure, pulse and weight every 3 to 6 months, and height every 6 months in children. Review need at least yearly. Ask about appetite, sleep, mood and misuse.

Stopping. No taper is needed; stop with review because mood and attention may worsen.

Referral and escalation. Refer to a specialist for diagnosis, initiation and complications. Chest pain, syncope, psychosis or fever with agitation needs urgent assessment.