When lay Buddhists “take refuge,” the Buddhist equivalent of confirmation, bar mitzvah, and similar ceremonies affirming one’s belief, they agree to attempt to live in conformity with the Five Precepts. (Some sects have eight, ten and even more, but the minimum is five.)
These precepts can be, and have been, translated in many ways in the 2600 years since the Buddha taught. Details vary, but a fair translation of the Fifth Precept (always the fifth, regardless of the total number of precepts) would be:
The basis of Buddhist philosophy is mindfulness, the opposite of carelessness. This is the idea that we should live our lives with attention, in order to live them well. Clearly, the use of alcohol or drugs that cloud the mind would not be conducive to mindfulness.
However, the precepts are considered to be guidelines. Buddhists do not recognize the concepts of sin and punishment for sin. Rather they believe in karma, the idea that bad behavior leads to bad experiences. They consider life and practice to be one and the same, and that both are composed of effort to attain perfect clarity of mind. Thus, one who fails to live up to the precept would create his own difficulties.
So, the answer to your question, in context, is that Buddhists attempt to refrain from all behavior that clouds the mind, but if they fail to do so they are perceived as creating their own consequences — clouded mind and whatever else — which are obstacles along the path to the goal of complete clarity.
If this makes it sound like no answer, that is the answer. Each person’s behavior is what it is, and it is not up to the rest of us to label it as acceptable or not, in the context of Buddhism.
However, in simple terms, it is thought best that if they do drink, they should avoid intoxication.