Symptoms
Acute overdosage may produce acute hypotension with other responses attributable to brain and gastro-intestinal malfunction (excessive sedation, weakness, bradycardia, dizziness, light-headedness, constipation, distension, flatus, diarrhoea, nausea, and vomiting).
Management
If ingestion is recent, emesis may be induced or gastric lavage performed. There is no specific antidote. Methyldopa is dialysable. Treatment is symptomatic. Infusions may be helpful to promote urinary excretion. Special attention should be directed towards cardiac rate and output, blood volume, electrolyte balance, paralytic ileus, urinary function and cerebral activity.
Administration of sympathomimetic agents may be indicated. When chronic overdosage is suspected, 'Kadomet' should be discontinued.
- depression
- on therapy with monoamine oxidase inhibitors (MAOIs)
- with a catecholamine-secreting tumour such as phaeochromocytoma or paraganglioma
- with porphyria.
None known.
Sedation, usually transient, may occur during the initial period of therapy or whenever the dose is increased. If affected, patients should not attempt to drive, or operate machinery. Headache, asthenia or weakness may be noted as early and transient symptoms.
The following convention has been utilised for the classification of frequency: Very common (>1/10), common (>1/100 and <1/10), uncommon (>1/1000 and <1/100), rare (> 1/10,000 and <1/1000), very rare (< 1/10,000) and not known (cannot be estimated from the available data).
| System Organ Class | Adverse event term | Frequency |
| Infections and infestations | Sialoadenitis | Not known |
| Blood and lymphatic system disorders | Haemolytic anaemia, bone-marrow failure, leukopenia, granulocytopenia, thrombocytopenia, eosinophilia | Not known |
| Endocrine disorders | Hyperprolactinaemia | Not known |
| Psychiatric disorders | Psychic disturbances including nightmares, reversible mild psychoses or depression, decreased libido | Not known |
| Nervous system disorders | Sedation (usually transient), headache, paraesthesia, Parkinsonism, VIIth nerve paralysis, choreoathetosis, mental impairment, carotid sinus syndrome, dizziness, symptoms of cerebrovascular insufficiency (may be due to lowering of blood pressure) | Not known |
| Cardiac disorders | Bradycardia, angina pectoris, myocarditis, pericarditis, atrioventricular block | Not known |
| Vascular disorders | Orthostatic hypotension (decrease daily dosage) | Not known |
| Respiratory, thoracic and mediastinal disorders | Nasal congestion | Not known |
| Gastrointestinal disorders | Nausea, vomiting, abdominal distension, constipation, flatulence, diarrhoea, colitis, dry mouth, glossodynia, tongue discolouration, pancreatitis | Not known |
| Hepatobiliary disorders | Liver disorders including hepatitis, jaundice | Not known |
| Skin and subcutaneous tissue disorders | Rash (eczema, lichenoid eruption), toxic epidermal necrolysis, angioedema, urticaria | Not known |
| Musculoskeletal and connective tissue disorders | Lupus-like syndrome, mild arthralgia with or without joint swelling, myalgia | Not known |
| Reproductive system and breast disorders | Breast enlargement, gynaecomastia, amenorrhoea, lactation disorder, erectile dysfunction, ejaculation failure | Not known |
| General disorder and administration site conditions | Asthenia, oedema (and weigh gain) usually relieved by use of a diuretic. (Discontinue methyldopa if oedema progresses or signs of heart failure appear). Pyrexia | Not known |
| Investigations | Positive Coombs test, positive tests for antinuclear antibody, LE cells, and rheumatoid factor, abnormal liver-function tests, increased blood urea | Not known |
Reporting of suspected adverse reactions
Reporting suspected adverse reactions after authorisation of the medicinal product is important. It allows continued monitoring of the benefit/risk balance of the medicinal product. Healthcare professionals are asked to report any suspected adverse reactions via; the Yellow Card Scheme at www.mhra.gov.uk/yellowcard
No relevant information.
In the treatment of hypertension.
Pharmacotherapeutic group: antiadrenergic agents; ATC code C02AB
Mechanism of action
It appears that several mechanisms of action account for the clinically useful effects of methyldopa and the current generally accepted view is that its principal action is on the central nervous system. The antihypertensive effect of methyldopa is probably due to its metabolism to alpha-methylnoradrenaline, which lowers arterial pressure by stimulation of central inhibitory alpha-adrenergic receptors, false neurotransmission, and/or reduction of plasma renin activity. Methyldopa has been shown to cause a net reduction in the tissue concentration of serotonin, dopamine, epinephrine (adrenaline) and norepinephrine (noradrenaline).
Absorption
Absorption of oral methyldopa is variable and incomplete.
Distribution
Bioavailability after oral administration averages 25%.
Biotransformation
Peak concentrations in plasma occur at two to three hours, and elimination of the drug is biphasic regardless of the route of administration. Plasma half-life is 1.8 ± 0.2 hours.
Elimination
Renal excretion accounts for about two thirds of drug clearance from plasma.
Acquired haemolytic anaemia has occurred rarely; should symptoms suggest anaemia, haemoglobin and/or haematocrit determinations should be made. If anaemia is confirmed, tests should be done for haemolysis. If haemolytic anaemia is present, 'Kadomet' should be discontinued. Stopping therapy, with or without giving a corticosteroid, has usually brought prompt remission. Rarely, however, deaths have occurred.
Some patients on continued therapy with methyldopa develop a positive Coombs test. From the reports of different investigators, the incidence averages between 10% and 20%. A positive Coombs test rarely develops in the first six months of therapy, and if it has not developed within 12 months, it is unlikely to do so later on continuing therapy. Development is also dose-related, the lowest incidence occurring in patients receiving 1 g or less of methyldopa per day. The test becomes negative usually within weeks or months of stopping methyldopa.
Prior knowledge of a positive Coombs reaction will aid in evaluating a cross-match for transfusion. If a patient with a positive Coombs reaction shows an incompatible minor cross-match, an indirect Coombs test should be performed. If this is negative, transfusion with blood compatible in the major cross-match may be carried out. If positive, the advisability of transfusion should be determined by a haematologist.
Reversible leukopenia, with primary effect on granulocytes has been reported rarely. The granulocyte count returned to normal on discontinuing therapy. Reversible thrombocytopenia has occurred rarely.
Occasionally, fever has occurred within the first three weeks of therapy, sometimes associated with eosinophilia or abnormalities in liver-function tests. Jaundice, with or without fever, may also occur. Its onset is usually within the first two or three months of therapy. In some patients the findings are consistent with those of cholestasis. Rare cases of fatal hepatic necrosis have been reported. Liver biopsy, performed in several patients with liver dysfunction, showed a microscopic focal necrosis compatible with drug hypersensitivity. Liver-function tests and a total and differential white blood-cell count are advisable before therapy and at intervals during the first six weeks to twelve weeks of therapy, or whenever an unexplained fever occurs.
Should fever, abnormality in liver function, or jaundice occur, therapy should be withdrawn. If related to methyldopa, the temperature and abnormalities in liver function will then return to normal. Methyldopa should not be used again in these patients. Methyldopa should be used with caution in patients with a history of previous liver disease or dysfunction.
Patients may require reduced doses of anaesthetics when on methyldopa. If hypotension does occur during anaesthesia, it can usually be controlled by vasopressors. The adrenergic receptors remain sensitive during treatment with methyldopa.
Dialysis removes methyldopa; therefore, hypertension may recur after this procedure.
Rarely, involuntary choreoathetotic movements have been observed during therapy with methyldopa in patients with severe bilateral cerebrovascular disease. Should these movements occur, therapy should be discontinued.
Interference with laboratory tests:
Methyldopa may interfere with the measurement of urinary uric acid by the phosphotungstate method, serum creatinine by the alkaline picrate method, and AST (SGOT) by colorimetric method. Interference with spectrophotometric methods for AST (SGOT) analysis has not been reported.
As methyldopa fluoresces at the same wavelengths as catecholamines, spuriously high amounts of urinary catecholamines may be reported interfering with a diagnosis of catecholamine-secreting tumours such as phaeochromocytoma or paraganglioma.
It is important to recognise this phenomenon before a patient with a possible phaeochromocytoma is subjected to surgery. Methyldopa does not interfere with measurements of VMA (vanillylmandelic acid) by those methods which convert VMA to vanillin. Methyldopa is contraindicated for the treatment of patients with a catecholamine-secreting tumour such as phaeochromocytoma or paraganglioma.
Rarely, when urine is exposed to air after voiding, it may darken because of breakdown of methyldopa or its metabolites.
'Kadomet' may cause sedation, usually transient, during the initial period of therapy or whenever the dose is increased. If affected, patients should not carry out activities where alertness is necessary, such as driving a car or operating machinery.
Posology
Use in adults:
Initial dosage: Usually 250 mg two or three times a day, for two days.
Adjustment: Usually adjusted at intervals of not less than two days, until an adequate response is obtained. The maximum recommended daily dosage is 3 g.
Many patients experience sedation for two or three days when therapy with 'Kadomet' is started or when the dose is increased. When increasing the dosage, therefore, it may be desirable to increase the evening dose first.
Withdrawal of 'Kadomet' is followed by return of hypertension, usually within 48 hours. This is not complicated generally by an overshoot of blood pressure.
Patients with renal impairment:
Methyldopa is largely excreted by the kidney, and patients with impaired renal function may respond to smaller doses.
Other antihypertensives:
Therapy with 'Kadomet' may be initiated in most patients already on treatment with other antihypertensive agents by terminating these antihypertensive medications gradually, as required. Following such previous antihypertensive therapy, 'Kadomet' should be limited to an initial dose of not more than 500 mg daily and increased as required at intervals of not less than two days.
When methyldopa is given to patients on other antihypertensives the dose of these agents may need to be adjusted to effect a smooth transition.
When 500 mg of 'Kadomet' is added to 50 mg of hydrochlorothiazide, the two agents may be given together once daily.
Paediatric population:
Initial dosage is based on 10 mg/kg of bodyweight daily in 2-4 oral doses. The daily dosage is then increased or decreased until an adequate response is achieved. The maximum dosage is 65 mg/kg or 3.0 g daily, whichever is less.
Older people:
The initial dose in elderly patients should be kept as low as possible, not exceeding 250 mg daily; an appropriate starting dose in the elderly would be 125 mg b.d. increasing slowly as required, but not to exceed a maximum daily dosage of 2 g. Syncope in older patients may be related to an increased sensitivity and advanced arteriosclerotic vascular disease. This may be avoided by lower doses.
Method of administration
Oral.
None.
