γ-Aminobutyricacid (/ˈɡæmə əˈmiːnoʊbjuːˈtɪrɨk ˈæsɨd/; or GABA /ˈɡæbə/) is the chief inhibitory neurotransmitter in the mammaliancentral nervous system. It plays a role in regulating neuronal excitabilitythroughout the nervous system. In humans, GABA is also directly responsible forthe regulation of muscle tone.

Althoughin chemical terms it is an amino acid, GABA is rarely referred to as such inthe scientific or medical communities, because the term "amino acid,"used without a qualifier, by convention refers to the alpha amino acids, whichGABA is not, nor is it considered to be incorporated into proteins.

Inspastic diplegia in humans, GABA absorption becomes impaired by nerves damagedfrom the condition's upper motor neuron lesion, which leads to hypertonia ofthe muscles signaled by those nerves that can no longer absorb GABA.

A numberof commercial sources sell formulations of GABA for use as a dietarysupplement, sometimes for sublingual administration. These sources typicallyclaim that the supplement has a calming effect. These claims are not utterlyunreasonable given the nature of GABA in human sympatholysis, but GABA as atranquilizing agent, purely isolated in itself, is scientificallyunsubstantiated or only irregularly demonstrated. For example, there isevidence stating that the calming effects of GABA can be observed in the humanbrain after administration of GABA as an oral supplement.[48] However, there isalso more scientifically and medicinally relevant evidence that pure GABA doesnot cross the blood–brain barrier at therapeutically significant levels.[40]The only way to deliver GABA effectively is to circumvent the blood-brainbarrier. Indeed, there are a small, limited number of over-the-countersupplements that are derivatives of GABA, such as phenibut and picamilon.Picamilon combines niacin and GABA and crosses the blood–brain barrier as aprodrug that later hydrolyzes into GABA and niacin.