So far in this thread I'm not sure we've discussed the relative difficulty of a bad guy making a counterfeit or doing a dilution or substitution based on the form of the medication.
The counterfeits in articles referenced so far were liquids in vials, so that would require just printing a box and re-labeling a saline vial. They'd probably copy a batch or lot number from the real drug. That makes me wonder if you emailed a picture of the packaging, with batch number, to the manufacturer, if they'd comment on authenticity. Probably not, because the lawyers would tell them not to. But they could probably spot a fake pretty easily.
But what if it were a tablet, especially one with a delayed release coating. It would seem that to produce a counterfeit of something like that would be much more difficult than a vial of liquid. You'd have all of the packaging details to manage, many are packaged in blister cards, and you'd have to get the tablet pressed and coated properly. Not impossible, of course, but unless you're talking about a super high priced Rx, the business case doesn't make sense...you'd just buy it from the real manufacturer and mark it up.
My experience is that, unlike getting meds from a US pharmacy, the mail order pharmacies send the Rx in the packaging they get from the manufacturer. In other words, they don't get a huge bottle of pills and manually count-out a subset to sell to customers. Instead, they sell you a whole container that came from the manufacturer.