The farther a line is from the center, the worse the drug (generally) is.
Edit: People have pointed out numerous times that some displayed stimulants, though ranking fairly low here, have a large potential for accidental overdosage. Don't take it as gospel, everything has risks that can't be easily plotted.
The chart was made using data from this 2007 Lancet study. It's not perfect, as drug reactions vary wildly between people and the "social harm" green line incorporates the opinions of pigs. It's definitely one of the better risk assessment tools out there since it doesn't incorporate ridiculous drug policies. Here is an alternate mapping.
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For many drugs, the "experimental" stage of use can rapidly enter dependency. Keep this in mind.
Made in reaction to this post where a comrade outlines family struggles with drug abuse, and how it's irresponsible to glorify drugs and alcohol. Use this chart to determine which drugs to glorify instead.
The study is essentially a short survey of medical experts, just a few pages long, and it doesn't get into the specific justifications for plotting drugs where they are.
In short, it found that surveyed medical and psychiatric professionals tend to closely agree on relative health and dependency risks. Those individual specialists probably considered various risks in administration for their own analyses (for example, the study mentioned that drugs involving needle use are especially bad), but those specific judgements are not covered extensively.
The study concerns itself with potential for drug abuse, so if something here is legally prescribed I'd recommend simply following your doctor's instructions and not worry too much about how it's parked by LSD in this graph. Not everything can be cleanly compared, many of these drugs are very different from others.