"In our Krsna consciousness movement we prohibit illicit sex, intoxication, meat-eating, and gambling. In Kali-yuga, a drunken, half-naked woman embracing a drunken man is a very common sight, especially in the Western countries, and restraining oneself after seeing such things is very difficult. Nevertheless, if by the grace of Krsna a man adheres to the regulative principles and chants the Hare Krsna mantra, Krsna will certainly protect him. Indeed, Krsna says that His devotee is never vanquished (kaunteya pratijanihi na me bhaktah pranasyati). Therefore all the disciples practicing Krsna consciousness should obediently follow the regulative principles and remain fixed in chanting the holy name of the Lord. Then there need be no fear. Otherwise one's position is very dangerous."
A Second Chance, Chapter 12
"Even by common sense we can understand. It requires little cool brain. But that cool brain cannot act without giving us, giving up these four things, namely illicit sex, meat-eating, intoxication, and gambling. If your brain is congested always with all these four rubbish things, you cannot think of higher, finer things. That is not possible. Therefore we restrict, to make the brain clear to understand about Krsna"
Srila Prabhupada Lecture on Bhagavad-Gita, 08-13-73, Paris
"Thereafter, all of them [the descendants of Vrsni and Bhoja], being permitted by the brahmanas, partook of the remnants of prasada and also drank liquor made of rice. By drinking they all became delirious, and, bereft of knowledge, they touched the cores of each other's hearts with harsh words.
"Purport: In ceremonies when brahmanas and Vaisnavas are sumptuously fed, the host partakes of the remnants of foodstuff after the guest has given permission. So the descendants of Vrsni and Bhoja formally took permission from the brahmanas and ate the prepared foodstuff. Ksatriyas are permitted to drink at certain occasions, so they all drank a kind of light liquor made of rice. By such drinking they became delirious and bereft of sense, so much so that they forgot their relationship with one another and used harsh words which touched the cores of each other's hearts. Drinking is so harmful that even such a highly cultured family becomes affected by intoxication and can forget themselves in a drunken state. The descendants of Vrsni and Bhoja were not expected to forget themselves in this way, but by the will of the Supreme it happened, and thus they became harsh towards one another."
"You are already intoxicated. You are already illusioned in this material world. If you still go on drinking, where is the possibility of knowledge? You must have sober brain to understand. So this is going on. Therefore my appeal to all the sober men: to understand this Krsna consciousness movement and join it for the benefit of his personal self, for the benefit of his country, for the benefit of the whole human society."
Srila Prabhupada Lecture, 11-10-73, Delhi
"What is the use of smoking and drinking tea and coffee? People are already intoxicated by material enjoyment, and if they indulge in further intoxication, what chance is there for self-realization? Similarly, one should not partake in gambling and unnecessarily agitate the mind. The real purpose of human life is to attain the spiritual platform and return to Godhead. That is the summum bonum of spiritual realization."
Caitanya-caritamrta, Madhya lila 7
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.