Amino acid

Acetyl-L-carnitine (ALCAR) is the acetyl ester of L-carnitine and the body's main carnitine ester. It is sold as a supplement and, in some countries, as a prescription drug. Meta-analyses find that it eases depressive symptoms, mostly in older adults and across small, inconsistent trials, and that it reduces diabetic nerve pain at doses above 1.5 g a day, on very low-certainty evidence. It did not slow Alzheimer's disease in two year-long trials, and it made chemotherapy nerve damage worse in a large one.

Creatine is one of the most researched sports supplements. It is best known for building strength and muscle, and is increasingly studied for the brain. Meta-analyses of randomized trials find small-to-moderate improvements in memory, attention and processing speed. The effects on memory are largest in older adults. It is well tolerated at 3 to 5 g a day.

DL-phenylalanine (DLPA) is a 50:50 mix of the dietary amino acid L-phenylalanine and its mirror image, D-phenylalanine. It is sold for mood and pain on the strength of two 1970s–80s ideas: that L-phenylalanine feeds catecholamine and phenylethylamine synthesis, and that D-phenylalanine slows the breakdown of enkephalins. The human evidence is old and thin. Small German depression studies were positive but had no placebo arm, and the only placebo-controlled chronic-pain trial of D-phenylalanine found no analgesic effect. The strongest modern data concern L-phenylalanine alone: short-term tolerability up to 12 g/day in healthy men and gut-hormone effects at 10 g.

L-theanine is an amino acid found almost only in tea. Taken at 200 to 400 mg a day, it may reduce stress and anxiety in people under stressful conditions. Its best-known use is alongside caffeine, where the pair improves attention switching more than either alone. A 2025 meta-analysis found small improvements in subjective sleep.

L-tyrosine is a dietary amino acid and catecholamine precursor, not a reliably demonstrated everyday nootropic. Small crossover trials suggest that large acute doses can preserve selected attention, working-memory or psychomotor outcomes during sleep loss, cold, noise or demanding multitasking, but reviews judge the evidence heterogeneous and too weak for a firm recommendation. Results in unstressed adults are mixed and baseline-dependent. A meta-analysis found no endurance benefit, although one later 12-person cycling experiment was positive. Four weeks at up to 4 g/day produced no dose-related safety signal in 30 healthy men, but that does not establish long-term or clinical-population safety.

N-acetyl-L-tyrosine (NALT) is a water-soluble acetylated derivative of L-tyrosine. Human evidence is almost entirely intravenous nutrition research, where 35–60% was recovered unchanged in urine and plasma tyrosine often did not rise. There are no cited human oral trials showing that NALT improves cognition, stress performance or energy.

N-acetylcysteine (NAC) is an acetylated form of the amino acid cysteine. It is the standard antidote for acetaminophen (acetaminophen) overdose, a mucolytic that modestly cuts flare-ups of COPD and chronic bronchitis, and a widely sold supplement. Its psychiatric uses rest on a mixed record: a positive trial in adolescents who used cannabis failed to repeat in adults, and large trials found no benefit for idiopathic pulmonary fibrosis or for protecting the kidneys during angiography. Only about a tenth of an oral dose reaches the blood.