Chloroform Heavy colourless liquid. Used as anaesthetic, and to render a victim unconsious. However, it's not easy to cholorform a sleeping victim without waking them, and it can take up to 15 minutes to knock the victim out. Victim often convulses as the drug takes effect. Skin blisters on face, where choloroform is in contact with the skin.
Prolonged inhalation leads to death by heart failure, and ingestion by heart and liver failure.
Chloral Hydrate is related. It is usually ingested and, in small doses, can knock someone out very quickly if, for example, placed in their drink. Quickly fatal in small doses, killing by interfering with the blood's capabilities to absorb oxygen. Faint smell of bitter almonds, though less than the mystery writers tell us. Metaldehyde Found in camping stove fuel and slug pellets.