Well, they know now. Try to buy 5 boxes of certain cold medicines at a reputable store like Walgreens or Osco even in states without any laws restricting the sale- you won't be able to do it. All of these sleazy convenience stores had these medicines behind the counter long before any laws were passed- they knew the drug addicts would steal them- and not because they catch more colds. Ask one of these convenient store clerks for a Chore Boy pad and he'll pull it out from under the counter. And if it happens to be a store where you're a regular customer, take a look at the clerk's look of amazement when you ask for a product that only drug addicts purchase at his store.
Yet I was able to buy 10 bottles of Sominex...not a blink. Who needs 10 boxes of sleeping pills except someone with bad intent?
Or who has a friend with a dog with allergies...and who knows that Sominex and Benadryl are the same medication.
Disclaimer: I'm not a physician, etc.
What I see happening...is that Bubba goes up to the cold medicine distributor in Chicago, and says he needs four cases per month of the stuff, and shipped to this store front. The store-front is Billy-Bob's bait shop and he simply picks up the cases and resells to his buddies throughout the state who need the stuff. And of course, you charge double for each bottle because you are a capitalist at heart.