Gamma-aminobutyric acid, or GABA, is the main inhibitory neurotransmitter in the central nervous system. It dampens neuronal firing, and several sedative drug classes, including benzodiazepines and alcohol, act by enhancing its effects.
Not every calming medicine works through GABA. Antihistamines produce sedation mainly by blocking histamine signalling in the brain instead, which is why their risk profiles differ.