Levodopa (L-DOPA) was first isolated from seedlings of Vicia faba by Marcus Guggenheim in 1913. Levodopa, a dopamine precursor, is an effective and well-tolerated dopamine replacement agent used to treat Parkinson's disease. Oral levodopa has been widely used for over 40 years, often in combination with a dopa-decarboxylase inhibitor carbidopa, which reduces many treatment complications, extending its half-life and increasing levodopa availability to the brain. Entacapone, a catechol-O-methyltransferase inhibitor, can also be used to improve the bioavailability of levodopa, especially when used in conjunction with a carbidopa.

Amantadine hydrochloride has pharmacological actions as both an anti-Parkinson and an antiviral drug. The mechanism by which amantadine exerts its antiviral activity is not clearly understood. It appears to mainly prevent the release of infectious viral nucleic acid into the host cell by interfering with the function of the transmembrane domain of the viral M2 protein. In certain cases, amantadine is also known to prevent virus assembly during virus replication. It does not appear to interfere with the immunogenicity of inactivated influenza A virus vaccine. The mechanism of action of amantadine in the treatment of Parkinson's disease and drug-induced extrapyramidal reactions is not known. Data from earlier animal studies suggest that amantadine hydrochloride may have direct and indirect effects on dopamine neurons. More recent studies have demonstrated that amantadine is a weak, non-competitive NMDA receptor antagonist (K1 = 10µM). Although amantadine has not been shown to possess direct anticholinergic activity in animal studies, clinically, it exhibits anticholinergic-like side effects such as dry mouth, urinary retention, and constipation. Amantadine was approved by the FDA in 1966 as a prophylactic agent against Asian influenza, and eventually received approval for the treatment of influenza virus A in adults. In 1969, it was also discovered by accident to help reduce symptoms of Parkinson's disease, drug-induced extrapyramidal syndromes, and akathisia.

Orphenadrine is an anticholinergic drug of the ethanolamine antihistamine class used to treat muscle pain and to help with motor control in Parkinson's disease but has largely been superseded by newer drugs. Orphenadrine binds and inhibits both histamine H1 receptors and NMDA receptors. It restores the motor disturbances induced by neuroleptics, in particular, the hyperkinesia. The dopamine deficiency in the striatum increases the stimulating effects of the cholinergic system. This stimulation is counteracted by the anticholinergic effect of orphenadrine. It may have a relaxing effect on skeletal muscle spasms and it has a mood elevating effect. Orphenadrine is indicated as an adjunct to rest, physical therapy, and other measures for the relief of discomfort associated with acute painful musculoskeletal conditions. Orphenadrine is an anticholinergic with a predominantly central effect and only a weak peripheral effect. In addition, it has mild antihistaminic and local anesthetic properties. Parkinson's syndrome is the consequence of a disturbed balance between cholinergic and dopaminergic neurotransmission in the basal ganglia caused by a decrease in dopamine. Orphenadrine restores the physiological equilibrium and has a favorable effect on the rigidity and tremor of Parkinson's disease and Parkinsonian syndromes. Adverse reactions of orphenadrine citrate are mainly due to the mild anticholinergic action of orphenadrine citrate and are usually associated with higher dosage. Dryness of the mouth is usually the first adverse effect to appear. When the daily dose is increased, possible adverse effects include tachycardia, palpitation, urinary hesitancy or retention, blurred vision, dilatation of pupils, increased ocular tension, weakness, nausea, vomiting, headache, dizziness, constipation, drowsiness, hypersensitivity reactions, pruritus, hallucinations, agitation, tremor, gastric irritation and rarely urticaria and other dermatoses

Benztropine is an anticholinergic used in the symptomatic treatment of all etiologic groups of parkinsonism and drug-induced extrapyramidal reactions (except tardive dyskinesia). Benztropine possesses both anticholinergic and antihistaminic effects, although only the former has been established as therapeutically significant in the management of parkinsonism. Benztropine's anticholinergic activity is about equal to that of atropine. Benztropine also inhibits dopamine reuptake via the dopamine transporter at nerve terminals. Benztropine is a selective M1 muscarinic acetylcholine receptor antagonist. It is able to discriminate between the M1 (cortical or neuronal) and the peripheral muscarinic subtypes (cardiac and glandular). Benztropine partially blocks cholinergic activity in the CNS, which is responsible for the symptoms of Parkinson's disease. It is also thought to increase the availability of dopamine, a brain chemical that is critical in the initiation and smooth control of voluntary muscle movement. Used as an adjunct in the therapy of all forms of parkinsonism and also for use in the control of extrapyramidal disorders due to neuroleptic drugs.

Trihexyphenidyl (Artane, Apo-Trihex, Parkin, Pacitane), also known as benzhexol and trihex has been in clinical usage for decades.It is an anticholinergic used in the symptomatic treatment of all etiologic groups of parkinsonism and drug induced extrapyramidal reactions (except tardive dyskinesia). Trihexyphenidyl possesses both anticholinergic and antihistaminic effects, although only the former has been established as therapeutically significant in the management of parkinsonism. Trihexyphenidyl is a selective M1 muscarinic acetylcholine receptor antagonist. It is able to discriminate between the M1 (cortical or neuronal) and the peripheral muscarinic subtypes (cardiac and glandular). Trihexyphenidyl partially blocks cholinergic activity in the CNS, which is responsible for the symptoms of Parkinson's disease. It is also thought to increase the availability of dopamine, a brain chemical that is critical in the initiation and smooth control of voluntary muscle movement. Trihexyphenidyl is indicated for the treatment of parkinson's disease and extrapyramidal reactions caused by drugs.

Apomorphine (brand names: Apokyn, Ixense, Spontane, Uprima) is indicated for the acute, intermittent treatment of hypomobility, “off” episodes (“end-of-dose wearing off” and unpredictable “on/off” episodes) in patients with advanced Parkinson’s disease. Apomorphine has been studied as an adjunct to other medications. It is a non-ergoline dopamine agonist with high in vitro binding affinity for the dopamine D4 receptor, and moderate affinity for the dopamine D2, D3, and D5, and adrenergic α1D, α2B, α2C receptors. The precise mechanism of action as a treatment for Parkinson’s disease is unknown, although it is believed to be due to stimulation of post-synaptic dopamine D2-type receptors within the caudate-putamen in the brain.

Pergolide is a long-acting dopamine agonist approved in 1982 for the treatment of Parkinson’s Disease. It is an ergot derivative that acts on the dopamine D2 and D3, alpha2- and alpha1-adrenergic, and 5-hydroxytryptamine (5-HT) receptors. It was indicated as adjunct therapy with levodopa/carbidopa in the symptomatic treatment of parkinsonian syndrome. It was later found that pergolide increased the risk of cardiac valvulopathy. The drug was withdrawn from the US market in March 2007 and from the Canadian market in August 2007. Pergolide stimulates centrally-located dopaminergic receptors resulting in a number of pharmacologic effects. Five dopamine receptor types from two dopaminergic subfamilies have been identified. The dopaminergic D1 receptor subfamily consists of D1 and D5 subreceptors and are associated with dyskinesias. The dopaminergic D2 receptor subfamily consists of D2, D3 and D4 subreceptors and has been associated with improvement of symptoms of movement disorders. Thus, agonist activity specific for D2 subfamily receptors, primarily D2 and D3 receptor subtypes, are the primary targets of dopaminergic antiparkinsonian agents. It is thought that postsynaptic D2 stimulation is primarily responsible for the antiparkinsonian effect of dopamine agonists, while presynaptic D2 stimulation confers neuroprotective effects. This semisynthetic ergot derivative exhibits potent agonist activity on dopamine D2- and D3-receptors. It also exhibits agonist activity on dopamine D4, D1, and D5, 5-hydroxytryptamine (5-HT)1A, 5-HT1B, 5-HT1D, 5-HT2A, 5-HT2B, 5-HT2C, α2A-, α2B-, α2C-, α1A-, α1B-, and α1D-adrenergic receptors. Parkinsonian Syndrome manifests when approximately 80% of dopaminergic activity in the nigrostriatal pathway of the brain is lost. As this striatum is involved in modulating the intensity of coordinated muscle activity (e.g. movement, balance, walking), loss of activity may result in dystonia (acute muscle contraction), Parkinsonism (including symptoms of bradykinesia, tremor, rigidity, and flattened affect), akathesia (inner restlessness), tardive dyskinesia (involuntary muscle movements usually associated with long-term loss of dopaminergic activity), and neuroleptic malignant syndrome, which manifests when complete blockage of nigrostriatal dopamine occurs. High dopaminergic activity in the mesolimbic pathway of the brain causes hallucinations and delusions; these side effects of dopamine agonists are manifestations seen in patients with schizophrenia who have overractivity in this area of the brain. The hallucinogenic side effects of dopamine agonists may also be due to 5-HT2A agonism. The tuberoinfundibular pathway of the brain originates in the hypothalamus and terminates in the pituitary gland. In this pathway, dopamine inhibits lactotrophs in anterior pituitary from secreting prolactin. Increased dopaminergic activity in the tuberoinfundibular pathway inhibits prolactin secretion. Pergolide also causes transient increases in somatotropin (growth hormone) secretion and decreases in luteinizing hormone (LH) concentrations. Pergolide is not available for use by humans in the United States, but approved for veterinary use; it was used in various other countries for the treatment of various conditions including Parkinson's disease, hyperprolactinemia, and restless leg syndrome. Pergolide in Europe was indicated for Parkinson's disease only when other dopaminergic agonist treatments had failed, and treatment had to be initiated by a neurologist. The label warned against using doses of more than 5mg a day, whether alone or in combination with levodopa. However the marketing of this drug finally stopped in France in May 2011 and sales elsewhere in Europe ceased eventually.

Methixene is a tertiary antimuscarinic with actions similar to those of atropine; it also has antihistaminic and direct antispasmodic properties. It is used for the symptomatic treatment of parkinsonism, including the alleviation of the extrapyramidal syndrome induced by other drugs such as phenothiazines, but, like other antimuscarinics, it is of no value against tardive dyskinesias. Metixene has been discontinued. Parkinsonism is thought to result from an imbalance between the excitatory (cholinergic) and inhibitory (dopaminergic) systems in the corpus striatum. The mechanism of action of centrally active anticholinergic drugs such as metixene is considered to relate to competitive antagonism of acetylcholine at muscarinic receptors in the corpus striatum, which then restores the balance.

Biperiden, sold under the brandname Akineton was used as an adjunct in the therapy of all forms of parkinsonism (postencephalitic, arteriosclerotic and idiopathic). Was also useful in the control of extrapyramidal disorders due to central nervous system drugs such as phenothiazines and other groups of psychotropics. Biperiden is a weak peripheral anticholinergic agent. It has, therefore, some antisecretory, antispasmodic and mydriatic effects. In addition, biperiden possesses nicotinolytic activity. Parkinsonism is thought to result from an imbalance between the excitatory (cholinergic) and inhibitory (dopaminergic) systems in the corpus striatum. The mechanism of action of centrally active anticholinergic drugs such as biperiden is considered to relate to competitive antagonism of acetylcholine at cholinergic receptors in the corpus striatum, which then restores the balance. Atropine-like side effects such as dry mouth; blurred vision; drowsiness; euphoria or disorientation; urinary retention; postural hypotension; constipation; agitation; disturbed behavior may been seen. Only limited pharmacokinetic studies of biperiden in humans are available.

Procyclidine is a muscarinic antagonist that crosses the blood-brain. Procyclidine hydrochloride (brand name Kemadrin) is a synthetic antispasmodic compound of relatively low toxicity. It has been shown to be useful for the symptomatic treatment of parkinsonism (paralysis agitans) and extrapyramidal dysfunction caused by tranquilizer therapy. Procyclidine hydrochloride was developed at The Wellcome Research Laboratories as the most promising of a series of antiparkinsonism compounds produced by chemical modification of antihistamines. Kemadrin is indicated in the treatment of parkinsonism including the postencephalitic, arteriosclerotic, and idiopathic types. Partial control of the parkinsonism symptoms is the usual therapeutic accomplishment. Procyclidine hydrochloride is usually more efficacious in the relief of rigidity than tremor; but tremor, fatigue, weakness, and sluggishness are frequently beneficially influenced. It can be substituted for all the previous medications in mild and moderate cases. For the control of more severe cases, other drugs may be added to procyclidine therapy as indications warrant. The mechanism of action is unknown. It is thought that procyclidine acts by blocking central cholinergic receptors, and thus balancing cholinergic and dopaminergic activity in the basal ganglia. Pharmacologic tests have shown that procyclidine hydrochloride has an atropine-like action and exerts an antispasmodic effect on smooth muscle. It is a potent mydriatic and inhibits salivation. It has no sympathetic ganglionblocking activity in doses as high as 4 mg/kg, as measured by the lack of inhibition of the response of the nictitating membrane to preganglionic electrical stimulation.