Alcohol suppresses vasopressin, a hormone regulating fluid, causing excess urination, resulting in extreme thirst, dry mouth, dizziness, and headache. The liver converts alcohol into acetaldehyde, a toxin many times more potent than alcohol, which causes nausea, vomiting, sweating, and rapid heart rate.
The immune system releases inflammatory agents (cytokines) throughout the body, leading to overall malaise, muscle aches, and fatigue, similar to fighting a virus. Gastritis occurs as alcohol irritates the stomach lining and increases acid production, causing nausea and pain. Metabolism of alcohol causes hypoglycemia, contributing to weakness, shakiness, and fatigue.
The brain struggles to rebalance neurotransmitters. After being suppressed, gamma-aminobutyric acid activity drops, while glutamate, a stimulant, rebounds, causing anxiety, jitters, and a low mood. Inflammation and metabolic changes impair attention, memory, and concentration.
Acetaldehyde dilates blood vessels in the head, causing pain, which is exacerbated by dehydration. Despite passing out, alcohol prevents REM sleep, leading to profound tiredness and lack of rest.
Alcohol makes it harder for the brain areas controlling balance, memory, speech, and judgment to do their jobs, resulting in a higher likelihood of injuries. In my early 20's I quit nicotine, it would be another twenty-five years before I managed to quit the demon drink.