system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does adding one methyl group to Adderall change it from being a common prescription medication to an extremely hard drug?
gpt
Why does adding one oxygen to water change it from being refreshing to a corrosive nightmare?

Even seemingly minor alterations to a chemical structure can radically alter the way it interacts with itself and other molecules.

In this particular case, the methyl group in *meth*amphetamine is altering the electron structure around the adjacent nitrogen center, which significantly accelerates its reactivity.