This is one reason my town voted to ban all retail MJ shops. The medical stuff is grown, tested and distributed to valid outlets to avoid such things. There are two companies that do this in the town I live in. They do not distribute here. How other places do it to accommodate the potheads, I can only imagine.
I legally grow my own for medical reason s. I never use fertilizers or herbicides, with the exception of organic potting soil, the rest is sunshine and well water.