People don't need to be protected against themselves. That's exactly what makes people not being able to care for themselves. Prohibition usually means no awareness is being raised about the risks or effects of a certain drug.
We get raised with the idea that drugs are bad and awful, that all of them are highly addictive and super deadly. Except for alcohol of course, please drink as much as you like, it's safe. This scare campaign works for most of us, but not for the lucky few who are curious and try it anyway. Guess what? They find out it's not bad, awful, deadly or addictive at all, but of course most of them are, still ignorant of all the other details of the drugs they used. So this first time drug user tells his friends that drugs aren't so bad at all. Some friends will probably try it as well.
If this concerns weed or mdma, this is not really a problem. These are safe to use, hard to overdose, make you a social human being and they are not (physically) addictive, but it might just as easy be cocaine, speed (meth), lsd or shrooms. The first two are addictive, bad for your body and brain and cocaine changes most people into irritating macho men while prolonged use of speed can make one paranoid. The latter two are psychedelics. These can be dangerous if you don't know what you're doing. These drugs open your mind, makes you realize things you never really thought about before and confronts you with unprocessed experiences or trauma's you may have, which can be quite scary, especially if you don't expect it.
Imagine your friend had a super duper happy lsd trip and recommends this to his depressed friend. Chances are high that he will go 'bad' resulting in psychosis. On the other hand, he might suddenly see the light and find out how he can get out of his depression.
If drugs were legal and awareness about their pro's, cons and dangers is being raised, more people will be encouraged to do some reading before they use something or at least the friend who brings the drugs knows what he's carrying and what the effects are.
People only need to be protected for misinformation, by informing them with facts instead of fear, uncertainty and doubt. Informed people make less (stupid) mistakes, and no one should be protected against themselves. If you die because of ignorance in an informed society, it's your own effing fault.
Drugs don't destroy people and communities, misinformation and ignorance does.