I do not understand this statement.  Are you implying weed use leads to heroin use?  (This has been proven to be false, marijuana is NOT a gateway drug).  Or are you somehow suggesting weed and heroin are the same?


Of the 4 chemicals you've mentioned only one is safe to use.


Caffeine:
The median lethal dose (LD50) given orally, is 192 milligrams per kilogram in rats.

Heroin:
Even for a non-user, the LD50 can be placed above 350 mg though some sources give a figure of between 75 and 375 mg for a 75 kg person.

Sugar:
Table sugar is sucrose, whose LD50 is 29,700 mg/kg

Weed:
NONE.  There is no known dose of marijuana that will kill a mammal.  Marijuana is not poisonous.