These substances include small amounts of chemicals such as methanol and other alcohols (known as fusel alcohols), acetone, acetaldehyde, esters, tannins, and aldehydes (e.g. furfural).
[1] Brandy, rum and red wine have the highest amount of congeners, while vodka and beer have the least.
There is some evidence that high-congener drinks induce more severe hangovers,[2][3] but the effect is not well studied and is still secondary to the total amount of ethanol consumed.
[4] Although methanol is not produced in toxic amounts by fermentation of sugars from grain starches,[5] it is a major occurrence in fruit spirits.
[6] However, in modern times, reducing methanol with the absorption of a molecular sieve is a practical method for production.