sedative sedative a sedative or tranquilliser is a substance that induces sedation by reducing irritability or excitement. they are cns depressants and interact with brain activity causing its deceleration. various kinds of sedatives can be distinguished, but the majority of them affect the neurotransmitter gamma-aminobutyric acid (gaba). in spite of the fact that each sedative acts in its own way, most produce relaxing effects by increasing gaba activity. sedativedrug classclinical datadrugs.comdrug classes