Amantadine

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Definition

Amantadine is a synthetic antiviral agent that also has strong antiparkinsonian properties. It is sold in the United States under the brand name Symmetrel, and is also available under its generic name.

Purpose

Amantadine is used to treat a group of side effects (called parkinsonian side effects) that include tremors, difficulty walking, and slack muscle tone. These side effects may occur in patients who are taking antipsychotic medications used to treat mental disorders such as schizophrenia . An unrelated use of amantadine is in the treatment of viral infections of some strains of influenza A.

Description

Some medicines, called antipsychotic drugs, that are used to treat schizophrenia and other mental disorders can cause side effects similar to the symptoms of Parkinson's disease. The patient does not have Parkinson's disease, but he or she may experience shaking in muscles while at rest, difficulty with voluntary movements, and poor muscle tone. These symptoms are similar to the symptoms of Parkinson's disease.

One way to eliminate these undesirable side effects is to stop taking the antipsychotic medicine. Unfortunately, the symptoms of the original mental disorder usually come back, so in most cases simply stopping the antipsychotic medication is not a reasonable option. Some drugs such as amantadine that control the symptoms of Parkinson's disease also control the parkinsonian side effects of antipsychotic medicines.

Amantadine works by restoring the chemical balance between dopamine and acetylcholine, two neuro-transmitter chemicals in the brain . Taking amantadine along with the antipsychotic medicine helps to control symptoms of the mental disorder, while reducing parkinsonian side effects. Amantadine is in the same family of drugs (commonly known as anticholinergic drugs) as biperiden and trihexyphenidyl .

Recommended dosage

Amantadine is available in 100-mg tablets and capsules, as well as a syrup containing 50 mg of amantadine in each teaspoonful. For the treatment of drug-induced parkinsonian side effects, amantadine is usually given in a dose of 100 mg orally twice a day. Some patients may need a total daily dose as high as 300 mg. Patients who are taking other antiparkinsonian drugs at the same time may require lower daily doses of amantadine (100 mg daily, for example).

People with kidney disease or who are on hemodialysis must have their doses lowered. In these patients, doses may range from 100 mg daily to as little as 200 mg every seven days.

Precautions

Amantadine increases the amount of the neurotransmitter dopamine (a central nervous system stimulant) in the brain. Because of this, patients with a history of epilepsy or other seizure disorders should be carefully monitored while taking this drug. This is especially true in the elderly and in patients with kidney disease. Amantadine may cause visual disturbances and affect mental alertness and coordination. People should not operate dangerous machinery or motor vehicles while taking this drug.

Side effects

Five to ten percent of patients taking amantadine may experience the following nervous system side effects:

dizziness or lightheadedness
insomnia
nervousness or anxiety
impaired concentration

One to five percent of patients taking amantadine may experience the following nervous system side effects:

irritability or agitation
depression
confusion
lack of coordination
sleepiness or nightmares
fatigue
headache

In addition, up to 1% of patients may experience hallucinations , euphoria (excitement), extreme forgetfulness, aggressive behavior, personality changes, or seizures . Seizures are the most serious of all the side effects associated with amantadine.

Gastrointestinal side effects may also occur in patients taking amantadine. Five to ten percent of people taking this drug experience nausea and up to 5% have dry mouth, loss of appetite, constipation, and vomiting. In most situations, amantadine may be continued and these side effects treated symptomatically.

One to five percent of patients taking amantadine have also reported a bluish coloring of their skin (usually on the legs) that is associated with enlargement of the blood vessels (called livedo reticularis). This side effect usually appears within one month to one year of starting the drug and subsides within weeks to months after the drug is discontinued. People who think they may be experiencing this or other side effects from any medication should tell their physician.

Interactions

Taking amantadine along with other drugs used to treat parkinsonian side effects may cause increased confusion or even hallucinations. The combination of amantadine and central nervous system stimulants (such as amphetamines or decongestants) may cause increased central nervous stimulation or increase the likelihood of seizures.

Resources

BOOKS

American Society of Health-System Pharmacists. AHFS Drug Information 2002. Bethesda: American Society of Health-System Pharmacists, 2002.

DeVane, C. Lindsay, Pharm.D. "Drug Therapy for Psychoses." In Fundamentals of Monitoring Psychoactive Drug Therapy. Baltimore: Williams and Wilkins, 1990.

Jack Raber, Pharm.D.