A general anaesthetic is a drug that brings about a reversible loss of consciousness. In formal use since 1842 - with the first public demonstration of ether to carry out surgery. (Though it's likely that alcohol, another chemical classed as an anaesthetic, was previously used, extensively, for similar purposes). Since then, many other compounds with similar effects have been discovered, and several are now in routine use as general anaesthetics. But the neurological pathways by which general anaesthetics work are still unclear.
“It has always been believed that general anaesthetics exert their effects (analgesia, amnesia, immobility) by modulating the activity of membrane proteins in the neuronal membrane. However, the exact location and mechanism of this action are still largely unknown although much research has been done in this area.” source: Wikipedia.
Note: A further anomaly has recently gained attention with regard to xenon. The (extremely inert) gas works efficiently as a general anaesthetic, with low toxicity. It's known to be an NMDA inhibitor but :
“It is not possible to state categorically which […] effects on ligand-gated and receptor-gated ion channels are causally linked to the anaesthetic action of xenon.”
Given its rarity, xenon is expensive for medical use - but it can be recovered (almost 100%) from exhalations.
See: Xenon: no stranger to anaesthesia British Journal of Anaesthesia 91 (5)
Observation The xenon phenomenon might warrant a separate item