I've worked at a convenience store in a very small town, watching people waste $40 on snack foods with an EBT/SNAP card. The $40 they spend there could buy them $70 or more in groceries, instead of the hours worth of junk-food they purchased when they enter glassy-eyed, slurring their speech, or scratching/tremoring so hard they have trouble swiping a card and entering their PIN on a keypad. These people are taking away VITAL resources from their families that NEED them, to fuel or sustain their own habits. Should we remove benefits for flagged families? NO! Should we use these flags to find the families, the children, most in need of help? YES!
Ultimately, to answer the question, if you are going to drug test ANYONE, you need to drug test EVERYONE...but do not hold the results against families that need assistance. Hold the people taking those tests personally responsible, and do not punish families or victims.