You have to do this very well to make a difference as a lethal dose can be as little as a few grains of sand. Like you need scientific style mortar and pestle and the patience and dexterity and knowledge to properly pulverize and mix and then a very fine scale for measuring doses not to mention the temptation that powdered product has for significantly more dangerous non-oral drug usage.

It is not in theory a bad idea but in practice perhaps not the most reasonable advice.