Here is a Dharma talk by Venerable Ledi Sayādaw:
In the Book of the Eights of the Gradual Sayings, the Buddha teaches:“Monks, drinking intoxicants, if done frequently or habitually, leads to rebirth in hell, rebirth as an animal or as a hungry ghost. At the very least, the result of this evil deed when born as a human being is that one becomes a mad person.”
Taking intoxicants, being unwholesome kamma, has two evil consequences: unfortunate rebirth in the future, and bad consequences in the present life. This means suffering in hell and long term bad effects in the present life. So the effect of taking intoxicants or drugs is quite serious as it leads to the four states of misery: hell, animals, ghosts, and demons.
The present evil effect, among others, is at least becoming mad. Among the consequences of insanity are neurosis, paranoia, schizophrenia, psychosis, etc. So the present results are also serious, just like future evil rebirths.
Why do intoxicants cause rebirth in hell in future lives? The reason is that drinking alcohol and taking drugs promotes fresh evil kamma. Evil kamma leads to hell. Moreover, by these evil deeds, the door of opportunity is open for past evil kammas to give appropriate effects. Thus one suffers in hell. Please note that everyone has done evil deeds in their previous existences so, at present, good deeds should be done.
New Evil Kamma
Taking intoxicants and non-medicinal drugs means making fresh evil kamma because it promotes greed, hatred, and delusion. It also supports wrong views, which urge one to do new evil kammas. It sustains and develops the five hindrances such as lust, anger, dullness, restlessness, and doubt. Intoxicants damage genes and chromosomes. They also burn the mind and consciousness. So taking intoxicants affects the mental realm, which has profound significance. Evil bodily and vocal acts follow as a result of drunkenness. Killing, lying, abusing, etc., are the usual results of taking alcohol.
Drunkards always do bodily, vocal, and mental misdeeds, which are the ten evil kammas. So taking alcohol increases daily the burden of future suffering in hell, and the burden of evil and guilt. There is little doubt that a drunkard will be reborn in one of the lower realms when he dies, since he accumulates evil kamma every day. He is demented in the present life. For countless lives he does other serious unwholesome kamma. So with counteractive kamma of this type, when he dies, he suffers in the lower realms. His burden of evil kamma is heavy. His misbehaviour is serious. So fresh evil deeds appear.
Past Evil Deeds
In past lives one has already done many kinds of evils while wandering in the rounds of existence, as one seldom hears the true Dhamma. These precious chances are very rare. Meeting Noble Ones is very rare, possible only once in a thousand or ten thousand existences. In past lives one did various evils and misdeeds in thought, speech, and deed. Lives of crime, guilt, and evil lead to the realms of suffering. The likelihood of this result is increased by drinking alcohol. The breaking of this moral precept means supporting other past evil deeds that one has already done. Past evil deeds resemble robbers and murderers who stay near a village to commit crimes. Transgression of this precept gives impetus to evil deeds: past, present, or future. So the evil kamma of taking alcohol resembles receivers of stolen goods who live in the village. Robbers get their chance due to the receivers of stolen goods. Villages are destroyed by them.
Likewise, as long as the evil of taking intoxicants remains, there will be chances for the previous evil deeds to mature. People have to take rebirth in various hells when past misdeeds mature. Everyone has also done some good deeds in their past lives, but these good deeds have no chance to mature due to the present evil kamma. If one dies without abandoning this evil, one will not escape from the results of the past evil kamma. Certainly one will suffer in hell. In this sense, drinking alcohol certainly brings about hell in the next life. Because serious past evil kamma take their chance, one reaches Roruva hell.