It’s a natural substance.

So are uranium and hemlock, arsenic and the coca plants producing cocaine.

Today’s weed has been progressively genetically engineered so that it doesn’t look like the marijuana of the 70’s, 80’s or even the 90’s.  It is no longer a “natural substance”  Even THC levels in marijuana that is sold legally are not standing up to testing. (Denver Post this week 3/9/14)