I shall focus on alcohol for two reasons: It is the most widely-used problem drug, and I am an alcoholic. Because of the first fact, some problem that is related to alcohol abuse is likely to affect almost everyone in our society. Because of the second fact, I can fully appreciate the power of addictive drugs. This does not mean that I preach abstinence. . .nor that I encourage drinking. The pragmatic approach to drugs is the same as any other potential problem area: Make an informed decision based on the true facts.
Alcohol is not a stimulant; it is a neural depressant. Alcohol slows down the nerve signals that control both the mind and the body. If the nervous system is sufficiently depressed, you lose consciousness (pass out); otherwise, you could drink enough to die then and there. Short of that, the depressing action of alcohol is selective. It acts first on the part of the nervous system that produces fear. This is why alcohol appears to be a stimulant; if it decreases one's inhibitions (fears), it may increase performance. However, it concurrently decreases one's judgment leading to a false feeling of doing better when actually doing worse. In any case, alcohol destroys brain cells which can never be replaced.
The loss of inhibition is the most destructive feature of alcohol from a social point of view. When drunk, people may say or do things of which they are very ashamed when sober. Date rape is one example. If you are intent on "tying one on," best you do it in a safe context. Or if you're with someone who is so inclined, either get away before it's too late or be prepared for the consequences. It is not just the driving drunk who is dangerous.
The most insidious feature of alcohol is that it is addicting. One can become dependent on alcohol as a psychological habit and as a physiological need. Let me briefly explain both of these effects. If you are feeling anxious about something, say an impending exam, you can reduce your anxiety by drinking alcohol. When you feel anxious about something else, say an argument with your girl/boy friend, you can reduce that anxiety by drinking alcohol. In sum, alcohol can be the way you learn to cope with anxiety. Usually, drinking alcohol actually makes matters worse. . .the time you spent drinking is time you didn't spend studying for the exam, and the loss of inhibitions may turn a squabble into a real fight. This leads to greater anxiety and greater use of alcohol. It can become a vicious circle of fear then alcohol, more fear then more alcohol, still more fear, and still more alcohol, etc. Psychological dependence is habitual use as a way to cope with problems.
Regardless of one's reason for drinking, one's body responds by compensating. As with any toxic substance, alcohol causes the body to develop greater tolerance to it. What this means is that it takes more and more alcohol to reach the same degree of ecstasy. A person who "can hold his/her liquor" may be admired but is actually well on the way to physiological addiction. This is because tolerance also implies dependence on alcohol. I can assure you that there's no fun in drinking when you have to drink to satisfy your body's needs. Alcoholic withdrawal is painful, and the addict does not drink to achieve ecstasy, s/he drinks to relieve withdrawal symptoms.
Addiction is a long-term effect of excessive use of alcohol, and it is easy to buttress the argument with short-term effects. For example, alcohol interferes with REM sleep which is important to consolidate that day's learning and to prepare the brain for new learning. Abstinence from mind-altering drugs is clearly ideal, in principle. In practice, prohibition doesn't work for many people. For them, responsible use is the realistic goal. Sound knowledge of the dangers of alcohol should lead to moderation.