I'm going to mostly point out that although this is an appealing idea: that we only get out of ourselves what we put in/etc./etc. there's no reason for it to be true. In fact, I'd expect that for some drugs, it's probably not true: if you're talking about a substance that has dramatic, longer term effects on the functioning of seratonin/dopamine systems in the brain.
Moving away from the kind of nebulous 'better person' realm and into a slightly more measurable/agreed upon "normal person" realm, it's patently untrue for those with nonstandard brain chemistry (and completely backed by scientific+medical consensus--drugs are a baseline treatment for bipolar disorder or schizophrenia), and so I think there's no reason why drug use should not improve the experience/behaviors of people with what our society has deemed 'normal' or 'normal-enough' brain chemistry. I don't think there's something so different about people with what are considered diseases with a larger genetic component which have lasted for centuries: they are likely caused by a combination of otherwise beneficial traits coming together in a less than beneficial way.
Also, since the diseases I mentioned often do involve long term maintenance medication taken on a daily basis (such that there's always some amount of it in a patient's bloodstream), I think it's worth also mentioning that there are treatments for otherwise drug resistant depression such as electroconvulsive therapy which are effective when used every few months or only once or twice. I don't think it's unreasonable to assert that there are likely compounds that have impacts on people's brain chemistry that last longer than the life of the drug in the bloodstream.
I'd like to note, though, that I'm not advocating going and taking psychedelic drugs in an attempt to better oneself. They're compounds which have debatable (and unresearched) longer term effects on those who imbibe them, and which have fairly established negative impact on people who take them and are at risk for developing a serious mental illness. (family history, etc.) I think that the appropriate thing to do is to pressure govt. and push society to allow for actual, well founded scientific research into the matter. (I say this as someone who has taken a psychedelic drug once, and wouldn't attribute any long term betterment or changes in personality/etc. to it. However, as someone fairly familiar with mental illness and its treatment, I think that it's reasonable to assert that drugs with strong impacts on brain chemistry--and certainly on the functioning of the seratonin system, which is often implicated in depression, bipolar disorder, and psychotic disorders might have beneficial uses in treatment of some individuals, and perhaps betterment of those with more 'normal' brain chemistry.)