Asphyxiation is a severe, life-threatening condition in which the body does not receive adequate oxygen in order to function, typically caused by the air not getting through to the individual's lungs. There are several different ways that drug or alcohol use can be directly or indirectly related to someone dying of asphyxiation.
One way is through the person's airways being blocked, for example, by "pulmonary aspiration" -- inhaling vomit -- which is more likely under the influence of alcohol and drugs. Alcohol in particular has a tendency to produce large amounts of liquid vomit, and when people are intoxicated, they are less aware of what is happening to their bodies, and may fall more deeply asleep. This was the cause of death for the famous musicians Jimi Hendrix and Bon Scott, lead singer of ACDC.
Another type of asphyxiation happens when people overdose on a drug such as heroin, and their breathing becomes extremely shallow, so they don't take in enough oxygen to sustain them. Others cease breathing as a result of a seizure, which can happen as part of drug or alcohol withdrawal.
A different type of asphyxiation occurs when the person's breathing is okay, but the air that they breathe contains inadequate or contaminated oxygen -- such as by inhaling air containing carbon monoxide. This can happen accidentally, for example, through poorly maintained heating systems -- a high risk for people with financial problems, which can be an aspect of addiction, or it can happen deliberately, for example, through the inhalation of exhaust fumes as a method of suicide or homicide.
Various forms of choking also cause asphyxiation, and again, sometimes this is accidental, and sometimes deliberate, either as a method of suicide or homicide, or for erotic arousal. This latter type of asphyxiation may have been the cause of death of famous actor David Carradine. The need to seek excitement by using choking for erotic arousal may be an aspect of sex addiction for a minority of sex addicts.