An officer once said to me unless you grow your own marijuana, you don't really know what it is you are smoking. Even though most weed probably is not laced with anything, some assholes occasionally do that. (Few and one would assume more with legal junk as opposed to harder drugs.) When this conversation occurred, I ended up agreeing with the cop that I do not always know what I was smoking. Perhaps I should have said that wouldn't be the case if it was legal. People would then be able to grow their own and maintain that it's additive free. Then again perhaps it was for the best I didn’t say that. Obviously tampered weed is assumed to be easy to recognize in either appearance or effects. It seems that most people will come across many strains in their times smoking; each strain smells and slightly tastes differently. It seems that more smokers don't know what strain their bag is in comparison to smokers who could match up an exact name to their bag. It is clear that some bags are more appealing to other bags as well as some bags smelling funkier then other ones.. When one bag goes up in smoke, it seems hard to compare it to a new stash unless of course one has the option of several choices. Knowing oneself and how one reacts to specific substances seems important. Also another factor to consider in the purity of cannabis would be how willing the person getting cannabis would be to smoke it with you.