Stupid people take drugs. Stupid people also take risks. But some smart people take risks too, and stopping them from doing so is both patronizing and immoral. Some smart people take drugs.
Many drugs are illegal in the first world, though some, such as alcohol are not.
Why are drugs illegal? Do you have the right to stop someone else taking drugs? What if they want to take drugs? What if they are aware of the risks, and choose to take drugs anyway? Everywhere people take risks for entertainment - car racing, bike racing, parachuting, bungee jumping, abseiling, rock-climbing and even the seemingly idiot behavior of 'running with the bulls' in Spain. We accept these behaviors. We say that people have the right to take risks for their entertainment. We call it 'character building' or 'stress relieving' or something like that, but this is in fact what is going on.
However if someone chooses to take drugs for their entertainment we label them either as criminals (if we are conservatives) or as victims (if we are humanitarians).
In fact, the reason that there is so much opposition to taking drugs is because people who are affected by drugs are unproductive. They tend to be inactive, lethargic, slow, and happy to spend their time sitting around doing nothing. On the other hand, people who take risks in physical activities tend to be in productive jobs, actively interact with those around them.
Despite the attempts of bohemians to deprogram our thinking, most of us still think of productivity from an GDP viewpoint. Someone who works a full 40 hour week to go skiing all the weekend makes a high contribution to the nation's National Product. On the other hand someone who sits in a darkened room for most of their life, only working enough to pay for rent and food, and spending their time growing and smoking marijuana does not greatly contribute to National Product. They do not greatly enrich those around them by actively trading and, of course, they do not pay much tax.
By any rational measure, taking drugs is pretty similar an entertainment as skiing. Both involve significant risk, and both (according to those who choose to participate) involve significant interest and fun. The lethargic drug taker may have just a good a life by any rational measure, but he does not generate as good a life for those around him. Someone who does not trade (or who minimizes their trade by only working enough to pay rent and food) does not enrich 'society'.
Hence we stop people taking drugs, and therefore pressure them to take up expensive entertainments. By doing so we are making them slaves to the rest of us, and taking away their free choice. The action is immoral.
However we justify our war on drugs by telling ourselves how much happier people are when the are no longer taking drugs. We point to the isolated examples of people who have voluntarily changed streams in their life, and feel much better for the experience. In so doing we ignore those who just take their habits further underground. We ignore those who suffer dreadfully in our prisons.