L-DOPA (levodopa, 3,4-dihydroxy-L-phenylalanine). As a drug it is used to treat parkinsonism. L-DOPA is able to pass the blood-brain barrier as a prodrug and is decarboxylated in the brain to the neurotransmitter dopamine by the enzyme aromatic-L-amino-acid decarboxylase. In this way, L-DOPA can relieve some of the dopamine deficit seen in parkinsonism. A possible side effect of L-DOPA is a condition similar to amphetamine psychosis.

L-Dopa is produced from the amino acid tyrosine by the enzyme tyrosine hydroxylase. It is also the precursor molecule for the catecholamine neurotransmitters dopamine and norepinephrine (noradrenaline), and the hormone epinephrine (adrenaline).

In work that earned him a Nobel Prize, Swedish scientist Arvid Carlsson first showed in the 1950s that administering L-DOPA to animals with Parkinson's Disease-like symptoms would cause a reduction of the symptoms. The neurologist Oliver Sacks describes this treatment in human patients in his book Awakenings on which the movie of the same name is based.