Does Symmetrel (amantadine) cause side effects?

Symmetrel (amantadine) is an antiviral drug used to prevent and treat infections with influenza A. Symmetrel is also used to control the symptoms of Parkinson’s disease and for the treatment of drug-induced extrapyramidal symptoms. 

Symmetrel can inhibit the replication of viruses in cells. It was initially used to prevent influenza A during flu season, and, if given within 24 to 48 hours of the onset of flu symptoms, to decrease the severity of the flu. Later Symmetrel was found to cause improvement in the symptoms of Parkinson’s disease.

Symmetrel's mechanism of action in Parkinson's disease is not fully understood. Symmetrel is less effective than levodopa in Parkinson's disease but can offer additional benefit when taken with levodopa. The brand name Symmetrel is discontinued.

Common side effects of Symmetrel include:

Serious side effects of Symmetrel include:

Drug interactions of Symmetrel include other drugs that can cause drowsiness such as:

Symmetrel may also interact with haloperidol, metoclopramide, and phenothiazines, which can block dopamine in the brain, and diuretics, which can reduce the kidney's ability to eliminate Symmetrel.

No well-controlled studies have been done in pregnant women to evaluate Symmetrel’s safety. Physicians may choose to use Symmetrel during pregnancy when the potential benefits outweigh the potential but unknown risks to the fetus.

Symmetrel is excreted into breast milk in low concentrations. Although no information is available on the effects in infants, the manufacturer recommends Symmetrel be used cautiously in breastfeeding women.

What are the important side effects of Symmetrel (amantadine)?

The most frequent side effects associated with amantadine that can appear after a few hours or several days of therapy include:

Less common side effects include:

Symmetrel (amantadine) side effects list for healthcare professionals

The adverse reactions reported most frequently at the recommended dose of Symmetrel (amantadine hydrochloride) (5-10%) are:

  • nausea,
  • dizziness (lightheadedness), and
  • insomnia.

Less frequently (1-5%) reported adverse reactions are:

Infrequently (0.1-1%) occurring adverse reactions are:

Rare (less than 0.1%) occurring adverse reactions are:

Other adverse reactions reported during postmarketing experience with Symmetrel (amantadine hydrochloride) usage include:

Nervous System/Psychiatric

coma, stupor, delirium, hypokinesia, hypertonia, delusions, aggressive behavior, paranoid reaction, manic reaction, involuntary muscle contractions, gait abnormalities, paresthesia, EEG changes, and tremor. Abrupt discontinuation may also precipitate delirium, agitation, delusions, hallucinations, paranoid reaction, stupor, anxiety, depression and slurred speech;


cardiac arrest, arrhythmias including malignant arrhythmias, hypotension, and tachycardia;


acute respiratory failure, pulmonary edema, and tachypnea;




leukocytosis; agranulocytosis

Special Senses

keratitis and mydriasis;

Skin and Appendages

pruritus and diaphoresis;


neuroleptic malignant syndrome, allergic reactions including anaphylactic reactions, edema, fever, pathological gambling, increased libido including hypersexuality, and impulse control symptoms.

Laboratory Test

elevated: CPK, BUN, serum creatinine, alkaline phosphatase, LDH, bilirubin, GGT, SGOT, and SGPT.

What drugs interact with Symmetrel (amantadine)?

  • Careful observation is required when Symmetrel (amantadine hydrochloride) is administered concurrently with central nervous system stimulants.
  • Agents with anticholinergic properties may potentiate the anticholinergic-like side effects of amantadine.
  • Coadministration of thioridazine has been reported to worsen the tremor in elderly patients with Parkinson's disease, however, it is not known if other phenothiazines produce a similar response.
  • Coadministration of Dyazide (triamterene/hydrochlorothiazide) resulted in a higher plasma amantadine concentration in a 61-year-old man receiving Symmetrel (Amantadine Hydrochloride, USP) 100 mg TID for Parkinson's disease. 
  • It is not known which of the components of Dyazide contributed to the observation or if related drugs produce a similar response.
  • Coadministration of quinine or quinidine with amantadine was shown to reduce the renal clearance of amantadine by about 30%.
  • The concurrent use of Symmetrel (amantadine hydrochloride) with live attenuated influenza vaccine (LAIV) intranasal has not been evaluated. However, because of the potential for interference between these products, LAIV should not be administered within 2 weeks before or 48 hours after administration of Symmetrel (amantadine hydrochloride), unless medically indicated.
  • The concern about possible interference arises from the potential for antiviral drugs to inhibit replication of live vaccine virus. Trivalent inactivated influenza vaccine can be administered at any time relative to use of Symmetrel (amantadine hydrochloride).

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Medically Reviewed on 5/28/2020
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Professional side effects and drug interactions sections courtesy of the U.S. Food and Drug Administration.