Alcohol

Alcohol is a chemical compound and can be used in alcoholic drinks or as a drug, among other things. A drink with a strong taste of alcohol can sometimes be used to hide the taste of a poison, especially if the person drinking it is note used to drink. Agatha Christie herself didn’t drink alcohol, but it is featured in many of her works. Several characters rise an eyebrow over Hercule Poirot's taste in alcohol.

The House of Lurking Death
Dennis Radclyffe dies after drinking a cocktail laced with ricin.

The Mirror Crack'd from Side to Side
The poisoned daiquiri that Heather Badcock drinks is believed to be intended for Marina Gregg.

Mrs McGinty's Dead
Hercule Poirot offers Superintendent Spence grenadine, Crème de menthe, Benedictine, Crème de Cacao or beer. Spence chooses beer.

Three Act Tragedy
Reverend Babbington dies after drinking a cocktail poisoned with nicotine.

The Clocks
Quentin Duguesclin is made unconscious through an alcoholic drink in which chloral hydrate had been added. The mixture is known as a "Mickey Finn". He is then stabbed to death.