Classifications: hormones and synthetic substitutes; enzyme inhibitor
Pregnancy Category: C


250 mg capsules


Blocks the enzyme tyrosine hydroxylase to inhibit the conversion of tyrosine to DOPA, which is the initial and rate-setting step in synthesis of catecholamines (dopamine, epinephrine, norepinephrine).

Therapeutic Effects

In patients with pheochromocytoma, reduces catecholamine synthesis as much as 80%, ameliorating hypertensive attacks and associated symptoms.


Short-term management of pheochromocytoma until surgery is performed, in long-term control when surgery is contraindicated, and in patients with malignant pheochromocytoma.

Unlabeled Uses

Has been used in selected patients with schizophrenia to potentiate antipsychotic effects of phenothiazines.


Control of essential hypertension. Safety during pregnancy (category C), lactation, or in children <12 y is not established.

Cautious Use

Impaired liver or kidney function.

Route & Dosage

Adult: PO 250 mg q.i.d., may increase to 2–3 g/d in divided doses (max: 4 g/d)



Adverse Effects (1%)

CNS: Sedation, fatigue; extrapyramidal signs: drooling, difficulty in speaking (dysarthria), tremors, jaw stiffness (trismus); frank parkinsonism, psychic disturbances (anxiety, depression, hallucinations, disorientation, confusion), headache, muscle spasms. GI: Diarrhea, nausea, vomiting, abdominal pain, dry mouth. Skin: Rash, urticaria. Urogenital: Transient dysuria, crystalluria, hematuria, impotence, failure of ejaculation. Endocrine: Breast swelling, galactorrhea. Body as a Whole: Peripheral edema, nasal stuffiness, shortness of breath. Hematologic: Eosinophilia.

Diagnostic Test Interference

False increases in urinary catecholamines may occur because of catechol metabolites of metyrosine.


Drug: Alcohol and other cns depressants add to sedation and CNS depression; droperidol, haloperidol, phenothiazines potentiate extrapyramidal effects.


Absorption: Readily absorbed from GI tract. Peak: 2–3 d. Duration: 3–4 d. Distribution: Crosses blood–brain barrier. Elimination: Excreted in urine. Half-Life: 3.4–7.2 h.

Nursing Implications

Assessment & Drug Effects

Patient & Family Education

Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug