Anyone who smoked pot like he did obviously did other drugs. Maybe you didn’t - it sounds like you indulged - but every stoner I’ve known (and I’ve known quite a few being a child of the 60s) has gone on to LSD and coke.
And Zero was not an occasional dabbler in pot... he was hardcore. He admitted in his “autobiography” that he did coke. There was a story about him doing “poppers” (I think that’s a name, whatever it is I have no idea) in connection with a homosexual tryst someone sort of witnessed.