Let me state my premises first.
  1. Marijuanna is less addictive than crack.

  2. Marijuanna is less destructive than heroine.

  3. Marijuanna does cause some impairment.

  4. The effects of marijuanna appear to be roughly comparable to those of alcohol.

  5. Some people may have stronger reactions to alcohol than to marijuanna or vice versa.
Now the problem is that items four and five are readily observable to anyone who sees someone use the two products. Items one and two are frequently denied by those opposing marijuanna use. Seeing this contradiction, it is reasonable to suppose that young people with no other sources of information than the ones misrepresenting items one and two would doubt what they say about item three.

Please explain how this example supports your position.