A friend of mine in Kentucky has a bourbon allergy. He drank bourbon all of his adult life and has developed an allergy to it (He's around 60 now.) It turns out when he drinks even small amounts of bourbon his heart develops an arrhythmia that can be life threatening. When he stops it goes away. After numerous tests and trials (drink or not drink) the doctor concluded it was that result of an allergy to bourbon. He now cannot drink bourbon at all. Clear liquor doesn't seem to have that effect.
Just thought it was interesting. Glad I didn't start bourbon till later in life.