The overwhelming majority of heroin overdoses are caused by combining heroin with alcohol or other drugs, especially sedatives.[1] This is because both heroin and alcohol are depressants, meaning they lower blood pressure, heart rate, and respiratory output (i.e.  slows breathing). In other words, heroin and alcohol bring bodily functions to a slow crawl and risk permanent cessation of systems altogether.[2]

According to the Centers for Disease Control and Prevention, addiction to alcohol often precedes heroin use:

People who are addicted to alcohol are twice as likely to become addicted to Heroin at some point in their lifetimes. People who are addicted to one of the substances will often begin to combine it with the other in order to increase the sedative effects of each drug and achieve a stronger high. Due to the ways that the two substances affect the brain, the intoxication that results from mixing them is much more intense than using just one of them on their own.[3]

Furthermore, users often experience greater intoxication when alcohol is mixed with narcotics like heroin, making it easier to drink to excess and risk alcohol poisoning.