Ethanol is toxic, and the body begins to dispose of it immediately upon its consumption. In the liver, the alcohol dehydrogenase enzyme converts ethanol into acetaldehyde, which is itself toxic. This is destroyed almost immediately by the aldehyde dehydrogenase enzyme, which converts it to acetate ions. NADH must be recycled to NAD for the disposal of ethanol to continue. If the amount of ethanol consumed is not great, the recycling can keep up with the disposal of ethanol.
Ethanol acts as a drug affecting the central nervous system.
Its behavioral effects stem from its effects on the brain and not on the muscles or senses themselves. It is a depressant, and depending on dose, can be a mild tranquilizer or a general anesthetic. It suppresses certain brain functions. At very low doses, it can appear to be a stimulant by suppressing certain inhibitory brain functions. Alcohol levels in the brain are difficult to measure, and so blood alcohol levels are used to assess degree of intoxication.