Opiates and alcohol
Opiates and alcohol are both downers. They slow down activity in your nervous system. This has several risks. First, they both cause your breathing to slow down and become shallower. This can cause you to pass out faster than if you only take 1 drug.
Especially if you first drank alcohol and then take opiates, this increases the chance of passing out. If you first take opiates and then drink (some) alcohol, you are less likely to drink a lot. But of course it is better not to combine. This combination can cause serious physical risks.