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Deaths have been reported from overdose with SYMMETREL (amantadine hydrochloride) . The lowest reported acute lethal dose was 1 gram. Because some patients have attempted suicide by overdosing with amantadine, prescriptions should be written for the smallest quantity consistent with good patient management.
Acute toxicity may be attributable to the anticholinergic effects of amantadine. Drug overdose has resulted in cardiac, respiratory, renal or central nervous system toxicity. Cardiac dysfunction includes arrhythmia, tachycardia and hypertension. Pulmonary edema and respiratory distress (including adult respiratory distress syndrome - ARDS) have been reported; renal dysfunction including increased BUN, decreased creatinine clearance and renal insufficiency can occur. Central nervous system effects that have been reported include insomnia, anxiety, agitation, aggressive behavior, hypertonia, hyperkinesia, ataxia, gait abnormality, tremor, confusion, disorientation, depersonalization, fear, delirium, hallucinations, psychotic reactions, lethargy, somnolence and coma. Seizures may be exacerbated in patients with prior history of seizure disorders. Hyperthermia has also been observed in cases where a drug overdose has occurred.
There is no specific antidote for an overdose of SYMMETREL (amantadine hydrochloride) . However, slowly administered intravenous physostigmine in 1 and 2 mg doses in an adult2 at 1- to 2-hour intervals and 0.5 mg doses in a child3 at 5- to 10-minute intervals up to a maximum of 2 mg/hour have been reported to be effective in the control of central nervous system toxicity caused by amantadine hydrochloride. For acute overdosing, general supportive measures should be employed along with immediate gastric lavage or induction of emesis. Fluids should be forced, and if necessary, given intravenously. The pH of the urine has been reported to influence the excretion rate of SYMMETREL (amantadine hydrochloride) . Since the excretion rate of SYMMETREL (amantadine hydrochloride) increases rapidly when the urine is acidic, the administration of urine acidifying drugs may increase the elimination of the drug from the body. The blood pressure, pulse, respiration and temperature should be monitored. The patient should be observed for hyperactivity and convulsions; if required, sedation, and anticonvulsant therapy should be administered. The patient should be observed for the possible development of arrhythmias and hypotension; if required, appropriate antiarrhythmic and antihypotensive therapy should be given. Electrocardiographic monitoring may be required after ingestion, since malignant tachyarrhythmias can appear after overdose.
Care should be exercised when administering adrenergic agents, such as isoproterenol, to patients with a SYMMETREL (amantadine hydrochloride) overdose, since the dopaminergic activity of SYMMETREL (amantadine hydrochloride) has been reported to induce malignant arrhythmias.
SYMMETREL (amantadine hydrochloride) is contraindicated in patients with known hypersensitivity to amantadine hydrochloride or to any of the other ingredients in SYMMETREL (amantadine hydrochloride) .
2. D.F. Casey, N. Engl. J. Med. 298:516, 1978.
3. C.D. Berkowitz, J. Pediatr. 95:144, 1979.
Last reviewed on RxList: 3/12/2009
This monograph has been modified to include the generic and brand name in many instances.
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