Amantadine

Amantadine is a small, cage-shaped molecule based on the adamantane framework, chemically 1-aminoadamantane. It is closely related to two other adamantane drugs, the antiviral rimantadine and the Alzheimer's medicine memantine ^[1]. Its rigid, fat-soluble structure allows it to cross into the brain, which underlies its effects on the nervous system.

The drug was first developed as an antiviral and was approved in the United States in the 1960s to prevent influenza A ^[2]. In 1968 a patient with Parkinson's disease noticed that her symptoms improved while she was taking amantadine for the flu; this chance observation prompted formal study and led, within a few years, to its approval as an antiparkinsonian medicine ^[2]. Decades later, an extended-release form was approved specifically for the treatment of levodopa-induced dyskinesia ^[3].

As an antiviral, amantadine works by blocking a protein of the influenza A virus called the M2 ion channel, a step the virus needs in order to replicate. Because this target is easily altered by a single mutation, resistant strains spread widely, and health authorities no longer recommend amantadine for the prevention or treatment of influenza ^[2]. It has no useful activity against influenza B.

In neurology, amantadine is valued for its dual action on the dopamine and glutamate systems. Randomized trials over the past several years have confirmed its usefulness in reducing levodopa-induced dyskinesia in Parkinson's disease, and it may also help smooth out motor fluctuations, although its benefit for drug-induced parkinsonism and other movement disorders is less well established ^[3]. It is used off-label for fatigue in multiple sclerosis and to promote recovery of consciousness and function after traumatic brain injury, though the supporting evidence there comes mostly from small studies ^[1][4]. Researchers continue to explore further uses, including in depression and other conditions ^[1].

Amantadine is well absorbed by mouth and is cleared largely unchanged by the kidneys, so doses must be lowered when kidney function is impaired ^[1]. It is a prescription medicine, available as immediate-release capsules and tablets, oral liquid, and extended-release formulations developed for Parkinson's disease ^[3].

Amantadine is prescription only. Distinctive effects include livedo reticularis, a harmless lace-like mottling of the skin, and swelling of the ankles. It can also cause insomnia, dizziness, dry mouth, blurred vision and, especially in older people, confusion or hallucinations. It should not be stopped abruptly, since that can worsen parkinsonism or rarely trigger a serious reaction. It is removed by the kidneys, so the dose is cut in kidney impairment, and it should be used carefully alongside other anticholinergic drugs, alcohol or other medicines that act on the brain.

Amantadine is routinely co-prescribed with levodopa and dopamine agonists in Parkinson disease treatment and enhances their therapeutic effects through a pharmacodynamic mechanism; amantadine's antiglutamatergic activity complements dopaminergic strategies ^[13]. Amantadine is renally cleared, and trimethoprim and other drugs that compete for renal tubular secretion theoretically increase amantadine levels, though specific controlled studies documenting this interaction are sparse in the published literature. Amantadine combined with intravenous morphine showed variable results across different studies depending on amantadine dosing, and a pharmacokinetic interaction was noted in some but not all randomized trials ^[14]. Drug interaction research for amantadine with most other antiviral, psychiatric, or cardiovascular agents remains limited.

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Amantadine, an adamantane derivative, was first introduced in the 1960s as an antiviral agent against influenza A, receiving United States Food and Drug Administration approval for that purpose in 1966. Its role in neurology was discovered by chance at the end of that decade, when a patient with Parkinson's disease taking amantadine to prevent influenza noticed a marked improvement in her parkinsonian symptoms, a serendipitous observation that redirected the drug's career. Over the following decades its antiviral use faded as influenza strains developed resistance, while its neurological applications grew. In 2017 an extended-release formulation, Gocovri, became the first drug specifically approved in the United States to treat levodopa-induced dyskinesia.

Amantadine has proven remarkably durable, remaining in common use by neurologists more than half a century after its serendipitous discovery in Parkinson's disease. It is particularly valued for its distinctive ability to reduce the involuntary movements, or dyskinesias, that arise from long-term levodopa therapy, a benefit supported by randomized controlled trials over the past decade and highlighted in a 2021 Lancet Neurology review. Its combination of dopaminergic and glutamatergic actions is considered pharmacologically unusual and underlies interest in additional uses such as fatigue and brain injury, though the evidence there is weaker. It is generally well tolerated, with side effects such as ankle swelling, livedo reticularis, and, in some patients, confusion or hallucinations.