The way I see it, the BackToTheFuturePart2 logic is that moving forward in time you remain in the same 'time-line' or 'time-path', just like you fast-forwarded... where-as traveling back in time necessarily jumps you to an alternate dimension the moment you arrive and start impacting things at any level.

If this logic were strictly adhered to I don't see how you could ever return to your original future once you've gone to the past... you'd always be returning to a new future impacted by the past you just messed with. ... something to think about before cranking that dial back to prehistory!

^you don't have your "life ruined by pot", rather, your life could be ruined by society's intolerance towards it. Or by your own inabilities to handle reality, thus you turn to the herb to escape. Also, as a physicist, you should probably be sensitive to causality. Which came first? The poor planning pothead or the weed that has been used by beneficial members of groups/societies since prehistory.

oh about permanently altering your brain chemistry, i'm sure it's more of a physical change than a chemical one. From what I know it only triggers chemicals that already exist and stimulate receptors. Once it's out of your brain, it goes back to normal.