3-O-methyldopa (3-OMD) is a metabolite of L-3,4-dihydroxyphenylalanine (L-DOPA), a drug used for the treatment of Parkinson's disease patients. 3-OMD is formed by catechol-O-methyltransferase. 3-OMD may be responsible for the side effects of L-DOPA.

Levonordefrin acts as a topical nasal decongestant and vasoconstrictor, most often used in dentistry. It is administered in a pre-mixed solution with local anesthetics, such as mepivacaine or procaine.

Methyldopa is an aromatic-amino-acid decarboxylase inhibitor in animals and in man. Methyldopa is a medication that has been used to treat high blood pressure since the 1960s. Methyldopa is indicated in the treatment of moderate to severe hypertension, including that complicated by renal disease. Only methyldopa, the L-isomer of alpha-methyldopa, has the ability to inhibit dopa decarboxylase and to deplete animal tissues of norepinephrine. D-isomer is relatively inactive. In man the antihypertensive activity appears to be due solely to the L-isomer, which became generally known as methyldopa (Aldomet). About twice the dose of the racemate (Methyldopa anhydrous, (±)-; DL-alpha-methyldopa) is required for equal antihypertensive effect. Racemic alpha-methyldopa was shown to be much less effective or ineffective for the treatment of hypertension. The comparative study of the hypotensive effect of L-alpha-methyl-dopa (L-isomer) versus the racemic form was performed. The short-term hypotensive effects of the racemic form and the L-isomer of alpha-methyl-dopa were compared in 13 hospitalized patients with arterial hypertension. After a placebo period the active preparations in a fixed dose of 1.5 g daily were administered for three-day periods separated by a second placebo period of three days, the sequence of the active drugs being alternated. Both substances were shown to exert significant hypotensive effects. The L-isomer produced significant blood-pressure reductions irrespective of whether or not it was given first, whereas the racemic form was effective only when given first. The blood-pressure levels obtained with the L-isomer were throughout lower than those with the racemic form. Methyldopa is a centrally acting antihypertensive agent. It is metabolized to alpha-methylnorepinephrine in the brain, and this compound is thought to activate central alpha-2 adrenergic receptors

Carbidopa is a competitive inhibitor of aromatic L-amino acid decarboxylase that does not cross the blood-brain barrier, is routinely administered with levodopa (LD) for the treatment of the symptoms of idiopathic Parkinson’s disease (paralysis agitans), postencephalitic parkinsonism, and symptomatic parkinsonism, which may follow injury to the nervous system by carbon monoxide intoxication and/or manganese intoxication. Current evidence indicates that symptoms of Parkinson’s disease are related to depletion of dopamine in the corpus striatum. Administration of dopamine is ineffective in the treatment of Parkinson’s disease apparently because it does not cross the blood-brain barrier. However, levodopa, the metabolic precursor of dopamine, does cross the blood- brain barrier, and presumably is converted to dopamine in the brain. When levodopa is administered orally it is rapidly decarboxylated to dopamine in extracerebral tissues so that only a small portion of a given dose is transported unchanged to the central nervous system. For this reason, large doses of levodopa are required for adequate therapeutic effect and these may often be accompanied by nausea and other adverse reactions, some of which are attributable to dopamine formed in extracerebral tissues. Carbidopa inhibits decarboxylation of peripheral levodopa. Carbidopa has not been demonstrated to have any overt pharmacodynamic actions in the recommended doses.

Methyldopate hydrochloride [levo-3-(3,4-dihydroxyphenyl)-2-methylalanine, ethyl ester hydrochloride] is the ethyl ester of methyldopa, supplied as the hydrochloride salt with a molecular weight of 275.73. Methyldopate hydrochloride is more soluble and stable in solution than methyldopa and is the preferred form for intravenous use. Methyldopate hydrochloride is an alpha adrenergic agonist that has both central and peripheral nervous system effects. Its primary clinical use is as an antihypertensive agent.