L-dopa (levodopa, dopamine precursor)

L-dopa is almost always given with a peripheral decarboxylase inhibitor (carbidopa or benserazide) that does not cross the blood-brain barrier. This matters: without it, AADC in the gut, liver, and blood vessels converts most L-dopa into peripheral dopamine before it reaches the brain, causing nausea and wasting the dose. The inhibitor prolongs L-dopa's plasma half-life and greatly increases the fraction that reaches the central nervous system.

A key pharmacokinetic nuance: L-dopa competes with other aromatic amino acids (tyrosine, tryptophan, phenylalanine) for the same transporter in the gut and at the blood-brain barrier. Protein-rich meals therefore reduce its absorption and brain entry. Vitamin B6 (pyridoxine) accelerates peripheral decarboxylation, so it is spaced apart from L-dopa taken without an inhibitor.

In a longevity and neuroprotection context, Mucuna pruriens supplies natural L-dopa alongside cofactors (endogenous decarboxylase inhibitors, NADH, CDP-choline) that in randomized trials against levodopa/benserazide have shown comparable efficacy with fewer dyskinesias. Any Parkinson's dosing must be supervised by a neurologist.