It seems like it's different for everybody (from what I've seen in life and read here). The variables seem to include baseline health and fitness (including mental health), nutrition, drink/drug of choice, other drugs being regularly taken, other issues of stress including work, family, finances etc., and sleep.
Don't be afraid that your head is going to explode if you quit and don't feel awful on day 1.
But I personally think a visit with an MD is very beneficial: frank talk, advice, bloodwork maybe, a plan and possibly a referral if judged to be useful.
Great reality check! And getting your MD on your side is generally beneficial, unless you have some judgmental old toad and then you should dump him anyway.