The oldest pharmacological approach to Parkinson's disease, now restricted to a narrow and carefully selected patient population.
Before levodopa transformed Parkinson's disease pharmacotherapy in the 1960s, anticholinergic drugs were the mainstay of treatment. Their mechanism targets the neurochemical imbalance in the striatum directly: dopamine deficiency produces relative cholinergic excess in the striatal interneuron network, and muscarinic blockade partially corrects this imbalance. Today their clinical role is narrow, but it remains real in appropriately selected patients.
The pathophysiological basis for anticholinergic efficacy in Parkinson's disease (PD) lies in the reciprocal relationship between dopaminergic and cholinergic striatal interneurons. Dopamine normally suppresses the tonically active cholinergic interneurons of the striatum, modulating acetylcholine release onto medium spiny neurons of both the direct and indirect pathways. When nigrostriatal dopamine is depleted, this suppression is lost and cholinergic interneuron activity increases, shifting the balance of striatal output in ways that amplify motor dysfunction. Muscarinic receptor antagonists, particularly those with M1 selectivity in the striatum, correct this imbalance by directly blocking the consequences of cholinergic excess.1
Three anticholinergic agents are used in clinical practice for PD in the United States: trihexyphenidyl, benztropine, and biperiden. Trihexyphenidyl is the most commonly prescribed; it is typically initiated at 1 mg daily and titrated slowly to 2–5 mg three times daily as tolerated. Benztropine (0.5–2 mg once or twice daily) is sometimes preferred for its longer duration of action and once-daily dosing convenience. Biperiden is less commonly used in the United States but more widely prescribed internationally. All three agents have similar therapeutic and adverse effect profiles, differing primarily in potency, duration, and the degree to which they penetrate the central nervous system (CNS).1
The clinical niche of anticholinergics in current PD practice is narrow and essentially restricted to two situations. First, they provide useful tremor suppression in young patients (typically under 70 years) with tremor-dominant PD who have minimal cognitive impairment and no significant urinary symptoms or glaucoma. Tremor in PD is notoriously resistant to levodopa at doses that adequately control bradykinesia and rigidity, and anticholinergics can provide incremental benefit where levodopa leaves tremor incompletely controlled. Second, drug-induced parkinsonism caused by dopamine-blocking agents (antipsychotics, metoclopramide) is frequently managed with anticholinergics, as the underlying mechanism is muscarinic-dopaminergic imbalance rather than nigrostriatal degeneration. Outside these two situations, anticholinergics are generally not appropriate in the modern antiparkinson armamentarium.2
Trihexyphenidyl: 1 mg/day titrated to 2–5 mg three times daily. Benztropine: 0.5–2 mg once or twice daily. Biperiden: less common in the US. All share M1 muscarinic blockade; differ in potency, duration, and CNS penetration. Clinical role: tremor in young, cognitively intact patients; drug-induced parkinsonism.
Why the risk-benefit calculus of anticholinergics shifts dramatically with age, and how to withdraw them safely in patients who have been on them for years.
The same muscarinic blockade that provides modest tremor control in young patients produces a constellation of adverse effects that are particularly hazardous in older patients and those with pre-existing cognitive impairment. Understanding this age-dependent risk profile is the key to appropriate prescribing and to recognizing which patients need to be tapered off anticholinergics they were started on years ago.
The peripheral adverse effects of anticholinergics follow directly from their mechanism: blockade of muscarinic receptors in the salivary glands (dry mouth), lacrimal glands (dry eyes), gastrointestinal smooth muscle (constipation, delayed gastric emptying), detrusor muscle (urinary retention), ciliary muscle (blurred near vision), and sweat glands (anhidrosis with risk of hyperthermia). These effects are consistent across the class. Dry mouth is the most common complaint and frequently limits dose escalation. Urinary retention is particularly dangerous in men with benign prostatic hyperplasia (BPH), where anticholinergics are essentially contraindicated. Narrow-angle glaucoma represents another absolute contraindication, as increased intraocular pressure from mydriasis can precipitate acute angle-closure crisis.3
The central adverse effects of anticholinergics are of greater clinical concern in PD, where the disease itself already impairs cholinergic neurotransmission in the cortex and hippocampus. Central muscarinic blockade produces a dose-dependent spectrum ranging from mild cognitive blunting and word-finding difficulty at lower doses, through vivid hallucinations and paranoid ideation at higher doses, to frank anticholinergic delirium at toxic doses. In patients with PD dementia or mild cognitive impairment, even low doses of anticholinergics can precipitate acute confusional states. The cumulative anticholinergic burden from multiple prescribed medications is increasingly recognized as a contributor to cognitive decline in older patients; prescribers managing PD patients should assess the total anticholinergic load across all medications, not just the specifically antiparkinson agents.34
Contraindications to anticholinergic use in PD are numerous and clinically important. Age over 70 years is a relative but strong contraindication in most guidelines. Cognitive impairment of any degree, including mild cognitive impairment (MCI), is a contraindication. BPH, narrow-angle glaucoma, significant constipation, tachyarrhythmias, and pre-existing dry mouth severe enough to compromise nutrition or dentition all preclude use. In practice, the combination of age, cognitive status, and comorbidity profile means that the majority of PD patients at any given time are not appropriate candidates for anticholinergic therapy.4
Deprescribing anticholinergics in patients who have been receiving them for years requires care. Abrupt discontinuation can precipitate rebound worsening of tremor and, in patients on long-term high doses, a withdrawal syndrome including nausea, sweating, and anxiety. The appropriate approach is slow tapering over weeks to months, reducing the dose by 25–50% every two to four weeks while monitoring for tremor worsening and withdrawal symptoms. If tremor worsens significantly on tapering, consideration should be given to whether another agent (propranolol, clonazepam, or adjustment of dopaminergic therapy) can provide alternative tremor control before anticholinergic discontinuation is complete.4
Age >70 years (relative but strong); any degree of cognitive impairment including MCI; benign prostatic hyperplasia; narrow-angle glaucoma; significant constipation; tachyarrhythmias. In practice the majority of PD patients at any point in their disease are not appropriate candidates. Cumulative anticholinergic burden across all medications must be considered when assessing any individual agent.
Never discontinue anticholinergics abruptly after prolonged use. Taper by 25–50% every 2–4 weeks. Monitor for tremor worsening and withdrawal symptoms (nausea, sweating, anxiety). If tremor worsens significantly, consider bridging with propranolol or clonazepam before completing taper.
How a repurposed antiviral became the first FDA-approved pharmacological treatment specifically for levodopa-induced dyskinesia, and what the extended-release formulation adds.
Amantadine was introduced as an antiviral agent in the 1960s and was found serendipitously to improve PD motor symptoms. Its mechanism involves uncompetitive antagonism at the N-methyl-D-aspartate (NMDA) receptor, the same glutamatergic pathway implicated in dyskinesia genesis. The extended-release formulation approved in 2017 exploits the pharmacokinetics of bedtime dosing to provide peak plasma concentrations during morning hours, when off periods and dyskinesia are typically most severe.
Amantadine's antiparkinson mechanism is multifactorial. It acts as an uncompetitive, low-affinity NMDA receptor antagonist, reducing the pathologically elevated glutamatergic drive in the basal ganglia that contributes to dyskinesia. It also has modest dopaminergic effects: it promotes dopamine release from surviving nigrostriatal terminals, inhibits dopamine reuptake, and has weak MAO inhibitory activity. In early PD, before significant nigrostriatal degeneration, this dopaminergic component may explain the modest symptomatic improvement seen when amantadine is used as early monotherapy. In advanced PD, however, the NMDA antagonist effect on dyskinesia is the clinically dominant mechanism.5
Immediate-release amantadine had been used off-label for dyskinesia management for decades, but its evidence base was limited and its pharmacokinetic profile was suboptimal. Amantadine extended-release (Gocovri, 137 mg once daily at bedtime) was approved by the FDA in 2017 specifically for the treatment of dyskinesia in patients with PD receiving levodopa-based regimens. The bedtime dosing strategy is pharmacokinetically deliberate: the extended-release formulation produces a broad plasma concentration peak during the morning hours, providing maximal NMDA antagonism at the time when dyskinesia and off-periods are typically at their worst. The EASE LID and EASE LID 3 trials demonstrated that amantadine ER 137 mg at bedtime significantly reduced UDysRS (Unified Dyskinesia Rating Scale) scores by approximately 27% compared to placebo and simultaneously reduced daily off time, without worsening on time.67
A second extended-release formulation, osmolex ER (129 mg once daily in the morning), was approved in 2018 for PD motor symptoms but does not carry the specific dyskinesia indication that Gocovri holds. The two formulations differ in their release profiles, dosing timing, and approved indications, and are not interchangeable. For dyskinesia management, Gocovri (bedtime dosing) is the clinically supported choice. The dose of Gocovri can be uptitrated from 68.5 mg to 137 mg after one week if tolerated. Renal dose adjustment is required: creatinine clearance 30–59 mL/min requires 68.5 mg at bedtime; creatinine clearance less than 30 mL/min or end-stage renal disease requires avoidance of the drug.6
The adverse effect profile of amantadine ER reflects both its glutamatergic and dopaminergic mechanisms. The most common adverse effects are hallucinations, dizziness, dry mouth, peripheral edema, constipation, and fall risk. Livedo reticularis, a mottled net-like skin discoloration of the lower extremities caused by cutaneous vasospasm, is a distinctive and reversible effect seen with both immediate-release and extended-release formulations; it is benign and resolves on discontinuation, but patients should be informed of it proactively. Suicidal ideation has been reported and is listed as a warning. Amantadine is renally cleared and accumulates in renal impairment. Abrupt discontinuation should be avoided; gradual tapering is preferred to prevent rebound worsening.56
Indication: Levodopa-induced dyskinesia in PD (first FDA-approved agent for this indication). Dose: 137 mg once daily at bedtime; start at 68.5 mg for first week. Mechanism: Uncompetitive NMDA antagonism + modest dopaminergic activity. Evidence: EASE LID trials: ~27% reduction in UDysRS vs placebo; off-time also reduced. Renal adjustment: CrCl 30–59: 68.5 mg; CrCl <30: avoid. Caution: hallucinations, livedo reticularis, fall risk; do not stop abruptly.
The first approved non-dopaminergic, non-glutamatergic mechanism for adjunct antiparkinson therapy, and what its unique pharmacology means for patient selection.
Istradefylline represents a mechanistically distinct approach to PD motor fluctuations. Rather than augmenting dopaminergic signaling or blocking glutamate receptors, it targets the adenosine A2A receptor, which is expressed selectively on the striatopallidal neurons of the indirect pathway. Blocking A2A receptors on these neurons reduces their inhibitory output, indirectly facilitating motor activation without directly stimulating dopamine receptors.
The adenosine A2A receptor is co-expressed with dopamine D2 receptors on striatopallidal medium spiny neurons of the indirect pathway. Under normal conditions, adenosine A2A receptor activation opposes D2 receptor signaling on these neurons, increasing their firing and augmenting indirect pathway inhibitory output to the globus pallidus interna, a net effect that suppresses motor activation. In the dopamine-depleted striatum of PD, this A2A-mediated indirect pathway overactivity is particularly pathogenic, contributing to the rigidity and akinesia of the off state. Istradefylline, by selectively blocking A2A receptors, reduces striatopallidal neuron firing and decreases indirect pathway overactivity, facilitating motor output without directly stimulating dopamine receptors.8
Istradefylline (Nourianz) was approved by the FDA in 2019 as an adjunct to levodopa/carbidopa in adults with PD experiencing off episodes. It is dosed at 20 mg once daily, with uptitration to 40 mg once daily permitted if additional benefit is needed and the lower dose is tolerated. It should be taken once daily at approximately the same time each day, without food restrictions. Hepatic metabolism via CYP3A4 is the primary elimination pathway; dose reduction to 20 mg once daily is required in patients with moderate hepatic impairment, and istradefylline is not recommended in severe hepatic impairment. Strong CYP3A4 inducers (rifampin, carbamazepine) significantly reduce istradefylline plasma concentrations and the combination should be avoided.89
The clinical trial evidence base for istradefylline consists of multiple phase 3 trials, most prominently the 6002-US-006 and 6002-US-013 studies, which demonstrated statistically significant reductions in daily off time of approximately 0.7–1.0 hours compared to placebo in levodopa-treated patients with motor fluctuations. A systematic review and meta-analysis of istradefylline trials confirmed a consistent reduction in off time across trials, with a pooled estimate of approximately 0.9 hours per day reduction versus placebo. Dyskinesia did not worsen meaningfully relative to placebo in these trials, a potential advantage over some dopaminergic adjuncts.9
The adverse effect profile of istradefylline is distinctive and clinically important. Dyskinesia is the most common adverse effect, occurring as a direct pharmacodynamic consequence of increased motor activation via the indirect pathway; this is typically mild and does not require drug discontinuation in most patients, but levodopa dose reduction may be needed. Hallucinations and impulse control disorders have been reported and carry a class warning; screening for pre-existing impulse control symptoms before initiation is appropriate. Istradefylline also has a notable interaction with smoking: tobacco smoking strongly induces CYP3A4, and smokers may require the 40 mg dose to achieve adequate plasma concentrations. The drug is classified as a Schedule V controlled substance in the United States due to its potential for dependence, a classification that should be noted when prescribing.89
Indication: Adjunct to levodopa/carbidopa for off episodes in PD. Dose: 20 mg once daily; uptitrate to 40 mg once daily if needed. Mechanism: Selective adenosine A2A receptor antagonism, reducing indirect pathway overactivity. Evidence: ~0.9 hr/day off-time reduction vs placebo across trials. CYP3A4: moderate hepatic impairment → 20 mg max; strong inducers (rifampin) → avoid; smokers may need 40 mg. Schedule V controlled substance.
Matching the mechanism to the clinical problem: a framework for choosing among anticholinergics, amantadine ER, and istradefylline in the context of the full antiparkinson regimen.
The agents covered in this module do not fit neatly into a linear treatment algorithm. Each targets a distinct mechanism and a distinct clinical problem. The practical skill is recognizing which clinical situation calls for which agent, and which patient characteristics predict benefit versus risk for each.
The decision to use an anticholinergic in PD reduces to a patient profile question: is the patient young (under 70), cognitively intact, free of BPH and glaucoma, and carrying a tremor-dominant phenotype that remains incompletely controlled on optimized dopaminergic therapy? If all of these conditions are met, a trial of trihexyphenidyl or benztropine is reasonable. If any of these conditions is not met (and in clinical practice, by mid-to-late stage PD the majority of patients fail on at least one), then anticholinergics should either not be started or, if already prescribed, should be systematically tapered off. The cognitive burden of anticholinergics in a disease that carries its own dementia risk is simply too high to justify routine use in unselected patients.4
Amantadine ER fills a specific niche: the levodopa-treated patient in whom dyskinesia has become functionally significant and who does not have severe hallucinations, significant renal impairment, or other factors that preclude its use. The key clinical point is that amantadine ER is not primarily a wearing-off drug; it reduces dyskinesia and also modestly reduces off time, but it should not be added to a regimen primarily to extend on time when the dominant problem is wearing-off. For wearing-off as an isolated complaint, COMT inhibitors, MAO-B inhibitors, and dose-interval adjustments of levodopa are the more appropriate primary interventions. Amantadine ER is best positioned as an adjunct specifically when dyskinesia is the limiting factor in optimizing the patient's levodopa dose.67
Istradefylline is positioned as an adjunct for levodopa-treated patients who continue to experience off episodes despite optimized dopaminergic therapy. Its non-dopaminergic mechanism offers a theoretical advantage in patients whose off episodes are not simply a consequence of insufficient levodopa exposure but reflect basal ganglia circuit dysfunction that responds to indirect pathway modulation. In practice, the off-time reduction of approximately 0.9 hours per day is modest compared to what is achievable with COMT or MAO-B inhibitors in ideal candidates, but the mechanistic difference means that istradefylline can be added on top of maximal dopaminergic adjunct therapy to provide further incremental benefit. The absence of significant worsening of dyskinesia in clinical trials is an advantage in patients already on the edge of dyskinesia threshold.9
The full adjunct pharmacology landscape of advanced PD now encompasses MAO-B inhibitors, COMT inhibitors, amantadine ER, and istradefylline, each with a distinct mechanism and a distinct clinical indication. Safe polypharmacy in this context requires keeping a clear accounting of what each added agent is intended to do, reassessing at each visit whether that goal is being achieved, and maintaining vigilance for the additive adverse effects of multiple agents with overlapping safety concerns, particularly hallucinations, orthostatic hypotension, impulse control disorders, and cognitive impairment, all of which can arise from more than one agent simultaneously.10
Tremor-dominant, young, cognitively intact: consider anticholinergic (trihexyphenidyl or benztropine). Dyskinesia as limiting problem: amantadine ER 137 mg at bedtime (Gocovri). Persistent off episodes on optimized dopaminergic therapy: istradefylline 20–40 mg once daily. Cognitive impairment present: anticholinergics contraindicated; amantadine ER and istradefylline require monitoring for hallucinations.
Multiple adjunct agents can produce overlapping adverse effects. Reassess at each visit: hallucinations (amantadine ER, istradefylline, dopamine agonists), orthostatic hypotension (levodopa, dopamine agonists), impulse control disorders (dopamine agonists, istradefylline), cognitive impairment (anticholinergics, amantadine ER). Every added agent should have a clear therapeutic goal and a plan for reassessment if that goal is not met within a defined time frame.