[S]cientists still don’t fully understand the causes of a hangover. (They do, however, have a scientific name for them: veisalgia.)
Most scientists believe that a hangover is driven by alcohol interfering with your body’s natural balance of chemicals in a more complex way.
Studies have found strong correlations between high levels of cytokines—molecules that the immune system uses for signaling—and hangover symptoms.
Normally, the body might use cytokines to trigger a fever of inflammatory response to battle an infection, but it seems that excessive alcohol consumption can also provoke cytokine release, leading to symptoms like muscle aches, fatigue, headache or nausea, as well as cognitive effects like memory loss or irritation.
You should not take acetaminophen (Tylenol) because when the liver is processing alcohol, it’s especially susceptible to acetaminophen’s toxic effects.
Via Smithsonian for the full article.