At the risk of freaking people out here... In college I used to go down to Tijuana and buy a years supply of birth control pills. I had my prescription - but it wasn't covered under insurance and in the early 80's it was over $20/month... Which was outside my college student budget. I could get the exact same pill package for $2/month. The only issue was that many pharmacies wouldn't have a full 12 packs of my brand/dosage, so I'd have to go to more than one pharmacy. I had a couple friends who'd go down with me - we'd all come back with a years supply each. I did this for 3 years and didn't get pregnant - nor did my friends...
I think we were more at risk from the street tacos we'd eat, than from bad birth control pills.
It would be harder today... passports required.
Manufacturing tablets, packaging them in tamperproof packaging, printing boxes and package inserts and distributing small lots to pharmacies is a relatively expensive set of activities for something that sells for a few dollars.
Grinding up lawn clippings, stuffing them in gelatin capsules and slapping an exotic appearing label on the front of a bottle of "Ancient Herbal Remedy #3" is much cheaper and more lucrative. You want counterfeit? That's where counterfeit is.