I have to wonder if this results in some kind of blackmarket trade for cancer drugs or other treatments that don't fall into the street drugs category. If it does, I am ignorant to it, likely because I am just beginning to understand (in my 40's, mind you) that the US health care system is so remarkably broken, my asthma treatments are governed not by my doctor, but whatever insurance my employer decides to carry. Lots of excellent replies here shedding some light on micro/macro ecenomics of the whole thing, but in a scary "how did it get like this" way.

It wouldn't be surprising. Back on the first Silk Road site, you could get more than just "fun drugs" / street drugs. Medical tourism has also been a thing for a long time. There was also some drama around insulin not that long ago, plenty of Americans went to Canada or Mexico for it.

Not just the US.

New Zealand has free health care, but there are a variety of drugs you can’t get prescribed easily (often because the system is trying to prevent drug-seeking). However it regularly ends up that necessary medicines are legally unobtainable (two situations recently in my own family). I have seen friends turn to illegal sources, or seek substitutes (especially “alternative” medicines and/or internet bullshit).

I live in former USSR, and here we have the opposite problem: everything (except for psychoactive drugs; for those you need to use black market) is on sale, open market essentualy. At any neighborhood pharmacy you can buy any drug, antibiotics, anticancer you name it, no prescription required. Which is also a very bad practice.

I illegally smuggled medication that I needed in from Canada when I didn't have health insurance in college. I had a prescription but couldn't afford US prices.

There are plenty of online prescription (no proof required) drug markets shipping generics from India, and yes, this does include cancer drugs.