I'm generally enthusiastic about nootropics, and more broadly about using pharmaceuticals (and other "external" stimuli) to affect the mind in various ways. However, I'm also concerned about the risk that such stimuli might have unknown negative long-term side effects. As a result, I would generally be hesitant to use or promote substances that are not both fairly well-tested, and have been around for a fairly long time, even if those substances have interesting short-term pharmacological profiles. I'm sure many other people are in a similar position.
However, I don't actually know what the baseline likelihood is that a new drug has negative long-term side effects, and I have not been able to find this statistic. This seems very relevant to pharmacology research, neurochemistry research, and the ethics of human trials, so I would be somewhat surprised if this information has not been compiled somewhere. Do you know where to find it?
I could make the question more specific like this (for example): when a new drug is discovered, what is the likelihood that it will later be discovered to have substantial negative side effects upon moderate but regular long-term use, conditional on there being no noteworthy negative short-term effects, and conditional on it not being chemically similar to any other drug that is known to have negative side effects under the same conditions? By "substantial negative side effect" I mean something like "a noteworthy negative effect that persists once the drug has been entirely metabolized, and that is not easily reversed". I would count things like "an increased risk of developing serious medical condition X" or "a permanent decrease in cognitive capacity Y", but not things like "a buildup of tolerance that induces a nasty withdrawal" (assuming there are no further issues after that).