compound
Fusel alcohols
Higher alcohols producing solvent-like heat and harshness, formed when fermentation runs too warm or under stress.
A moderate level contributes legitimate warmth to strong beers. Excess produces a hot, spirituous, nail-varnish character and is strongly associated with hangover severity, though that association is less well established than commonly claimed.
The main causes are high fermentation temperature, underpitching and inadequate wort oxygenation.