The causality is not obvious. For example, throughout the 70s and 80s, organizations shipping medicine and other supplies to Cuba were regularly bombed or attacked (e.g. Hispania Interamericana). Anti-Castro terrorist organizations (e.g. https://en.wikipedia.org/wiki/Alpha_66) operated openly in Miami.

So does Cuba lack medicine because they're communist? Or are they communist because the US worked against broad prosperity in Cuba, often very violently and explicitly, for decades before and after Castro?

Even before taking that into account, an embargo with what would otherwise be their most obvious trading partner seems like a big impediment.

There's always Canada, Netherlands, Mexico, Venezuela, Brazil, the west coast of Africa, etc.

Exactly. I never bought that argument. They can trade with everybody else, so that embargo doesn't have much of an impact.

Especially with the rise of containerized shipping. The US gets a lot of stuff from China and ships can get to Cuba for less than it costs to reach any city on the east coast of the US.

It's their rejection of the free market that makes them unable to procure goods in the free market. It's ironic that some people want the free market to serve a system that denies its citizens access to that same exact marketplace. They don't seem to question the ideas they are repeating.

Ok, that was 30 years ago. What's the explanation for why they haven't gotten aspirin to be readily available in the meantime?

Same reason why the "technical debt" never gets fixed... Got to ship features, man!