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Admin: PO/IV. Approx 20% bioavailability when given orally, probably due to first-pass hepatic metabolism
Elim: metabolised in liver by acetylation and thus half-life depends on acetylator status. Onset and duration of action depends on acetylator status


- direct arteriodilator with little venodilator action
- reduces diastolic pressure more than systolic
- may induce reflex tachycardia and increased cardiac output which may blunt its hypotensive effect. Combination with a central alpha-2 agonist or a beta blocker decreases this reflex sympathetic activity
- improves renal and uteroplacental blood flow in pre-eclampsia
- onset time 10-20 min. Duration of action 6-8 h

Adverse effects

- reflex tachycardia
- headaches,
- nausea & vomiting
- flushing
- skin rash
- lupus syndrome: more likely to develop after prolonged therapy, in slow acetylators and in patients with renal failure
- infusions may be difficult to titrate in pre-eclampsia and may be associated with a higher incidence of fetal distress
- in presence of hypovolaemia may result in hypotension and fetal distress
- neonatal thrombocytopenia (rare)


- in hypertensive emergencies: IV boluses of 10-20 mg, repeated as necessary at 15 min intervals, to a maximum of 50 mg. Can also be given as an infusion: 0.5-1 mg/min
- in pre-ecplamsia for control of BP: 5 mg IV initially followed by 5-10 mg every 20 min to maximum of 40 mg

Further reading

Chui PT, Low JM. Acute hypertension and vasodilators. In Oh TE (ed), Intensive Care Manual, 4th Ed., Butterworth Heinemann, Oxford, 1997, pp 153-62


© Charles Gomersall December 1999


©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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