The effects and dangers of barbiturate use increase greatly if they are taken with alcohol. Alcohol and barbiturates, such as phenobarbital, are both central nervous system depressants. Central nervous system depressants slow down the activity in the brain. If the nervous system is depressed too much, it can cease to function altogether which can lead to death. 

Taking both of them together causes a synergistic effect, which can result in acute toxicity. The acute effects of mixing barbiturates with alcohol include: Hypothermia, Respiratory depression, Lethargy, Loss of coordination and Low blood pressure. These effects arise due to decreased brain activity. It takes the brain longer to do its job, and as a result, it has difficulty regulating the body’s core functions, including blood pressure, breathing and temperature.